Note 4 - Stock-based Compensation (Details Textual) - USD ($) | 1 Months Ended | 3 Months Ended | 6 Months Ended |
Jul. 31, 2020 | Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2020 | Sep. 30, 2019 |
2006 Employee Equity Compensation Restricted Stock Plan (the "Employee Plan") [Member] | Restricted Stock [Member] | | | | | |
Stock Issued During Period, Shares, Restricted Stock Award, Gross (in shares) | | | | 972,175 | |
2016 Employee Equity Compensation Restricted Stock Plan [Member] | Restricted Stock [Member] | | | | | |
Stock Issued During Period, Shares, Restricted Stock Award, Gross (in shares) | | | | 221,798 | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period (in shares) | 68,190 | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value (in dollars per share) | $ 31.20 | | | | |
2016 Employee Equity Compensation Restricted Stock Plan [Member] | Restricted Stock [Member] | Chief Executive Officer [Member] | | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period (in shares) | 37,800 | | | | |
2006 Outside Director Equity Compensation Restricted Stock Plan (the "Director Plan") [Member] | Restricted Stock [Member] | | | | | |
Stock Issued During Period, Shares, Restricted Stock Award, Gross (in shares) | | | | 272,000 | |
2015 Outside Director Equity Compensation Restricted Stock Plan ("2015 Director Plan") [Member] | Restricted Stock [Member] | | | | | |
Stock Issued During Period, Shares, Restricted Stock Award, Gross (in shares) | | | | 172,500 | |
2015 Outside Director Equity Compensation Restricted Stock Plan ("2015 Director Plan") [Member] | Restricted Stock [Member] | Director [Member] | | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period (in shares) | 37,500 | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value (in dollars per share) | $ 31.20 | | | | |
Employee and Director Plans [Member] | | | | | |
Share-based Payment Arrangement, Expense | | $ 772,000 | $ 730,000 | $ 1,500,000 | $ 1,400,000 |
Employee and Director Plans [Member] | Restricted Stock [Member] | | | | | |
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount, Total | | $ 4,400,000 | $ 4,300,000 | $ 4,400,000 | $ 4,300,000 |
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition (Year) | | | | 3 years | 3 years |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number, Ending Balance (in shares) | | 177,876 | 191,000 | 177,876 | 191,000 |