Note 6 - Stock-based Compensation (Details Textual) - USD ($) shares in Thousands	12 Months Ended
Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period	5 years
Allocated Share-based Compensation Expense, Total	$ 40,920	$ 22,000	$ 21,160
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Dividend Rate	5.30%	7.00%	7.10%
Employee Service Share-based Compensation, Tax Benefit from Exercise of Stock Options	$ 8,953	$ 7,993
Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Number of Shares, Ending Balance	0	0
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Intrinsic Value	$ 34,850
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Intrinsic Value	482,300
Share-based Compensation Arrangement by Share Based Payment Award Options Grants in Period Fair Value	40,920
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Total	$ 0
Employee Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period	6 years 219 days	6 years 292 days	6 years 73 days
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period	1 year
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period	10 years

Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Fair Value

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Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

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Unrecognized cost of unvested share-based compensation awards.

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Amount of tax benefit from exercise of stock options granted under share-based compensation arrangement.

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Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

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Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding.

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Period from grant date that an equity-based award expires, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of vested portions of options outstanding and currently exercisable.

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Number of non-vested options outstanding.

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