Note 4 - Stock-based Compensation  | 
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| Notes to Financial Statements | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Disclosure of Compensation Related Costs, Share-based Payments [Text Block] | 
 Note  4.  Stock-Based CompensationThe Company recorded   $590,165  and $1,183,911  of compensation expense related to current and past option grants, restricted stock grants and the Company’s Employee Stock Purchase Plan (“ESPP”) for the three  and six  months ended  March  31,  2017,  respectively. For the three  months ended  March  31,  2017,  $535,464  of this expense is included in selling, general and administrative expense, and $54,701  is included in cost of sales. For the six  months ended  March  31,  2017,  $1,074,510  of this expense is included in selling, general and administrative expense, and $109,401  is included in cost of sales. The Company recorded $246,426  and $473,193  of compensation expense related to current and past equity awards for the three  and six  months ended  March  31,  2016,  respectively. For the three  months ended  March  31,  2016,  $224,156  of this expense was included in selling, general and administrative expense, and $22,270  was included in cost of sales. For the six  months ended  March  31,  2016,  $429,037  of this expense was included in selling, general and administrative expense, and $44,156  was included in cost of sales. As of  March  31,  2017,  $6,324,696  of total unrecognized compensation expense related to non-vested equity awards is expected to be recognized over a period of approximately 7.6  years.There were   no six  month periods ended  March  31,  2017  or  March  31,  2016.  The following is a summary of stock option activity during the six  months ended  March  31,  2017: 
 The intrinsic value of an option is the amount by which the fair value of the underlying stock exceeds its exercise price. As of    March  31,  2017,  the weighted average remaining contractual term for all outstanding and exercisable stock options was 3.2  years and their aggregate intrinsic value was $533,632.  During the six  months ended  March  31,  2017,  the Company received proceeds of $28,459  from the exercise of stock options. During the six  months ended  March  31,  2016,  exercised stock options totaled 57,050  shares, resulting in $84,738  of proceeds to the Company.Restricted Stock The Company’s   2007  Stock Compensation Plan permits its Compensation Committee to grant stock-based awards, including stock options and restricted stock, to key employees and non-employee directors. The Company has made restricted stock grants that vest over one  to ten  years.During the   six  month period ended  March  31,  2017,  the Company granted non-employee directors restricted stock awards totaling 3,795  shares of common stock, with a vesting term of approximately one  year and a fair value of $16.45  per share.During the   six  month period ended  March  31,  2016,  the Company granted non-employee directors restricted stock awards totaling 2,712  shares of common stock, with a vesting term of approximately one  year and a fair value of $14.73  per share. The Company also granted outgoing non-employee directors fully-vested stock awards totaling 1,356  shares of common stock, with a fair value of $14.73  per share. Additionally, the Company granted employees restricted stock awards totaling 8,500  shares of common stock, with a vesting term of five  years and a fair value of $13.64  per share during the six  month period ended  March  31,  2016. 
Restricted stock transactions during the   six  month period ended  March  31,  2017  are summarized as follows:
 Employee Stock Purchase Plan  Clearfield, Inc.’s ESPP allows participating employees to purchase shares of the Company’s common stock at a discount through payroll deductions. The ESPP is available to all employees subject to certain eligibility requirements. Terms of the ESPP provide that participating employees  
 may  purchase the Company’s common stock on a voluntary after-tax basis. Employees  may  purchase the Company’s common stock at a price that is no less than the lower of 85%  of the fair market value of one  share of common stock at the beginning or end of each stock purchase period or phase. The ESPP is carried out in six  month phases, with phases beginning on  January  1  and  July  1  of each calendar year. For the phases that ended on  December  31,  2016  and  December  31,  2015,  employees purchased 11,144  and 10,352  shares at a price of $15.21  and $11.40  per share, respectively. After the employee purchase on  December  31,  2016,  131,978  shares of common stock were available for future purchase under the ESPP. | 
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