Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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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