Note 9 - Stock Transactions and Stock-based Compensation - Restricted Stock Unit Activity (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
Mar. 31, 2022	Mar. 31, 2021	Mar. 31, 2020
Nonvested, weighted average remaining contractual life (Year)	4 years 3 months 18 days
Nonvested, aggregate intrinsic value	$ 14,093
Nonvested (in shares)	55
Nonvested, weighted average grant date fair value per share (in dollars per share)	$ 288.45
Restricted Stock Units (RSUs) [Member]
Awards granted, weighted average grant date fair value per share (in dollars per share)		$ 231.61	$ 213.31
Nonvested, weighted average grant date fair value per share (in dollars per share)	$ 251.94
Restricted Stock Units (RSUs) [Member] \| Equity Plan 2014 [Member]
Nonvested (in shares)	37
Nonvested, weighted average grant date fair value per share (in dollars per share)	$ 206.56
Nonvested, weighted average remaining contractual life (Year)	1 year	1 year 1 month 6 days
Nonvested, aggregate intrinsic value	$ 13,019	$ 8,948
Awards granted (in shares)	37
Awards granted, weighted average grant date fair value per share (in dollars per share)	$ 274.55
Awards forfeited or expired (in shares)	(3)
Awards forfeited or expired, weighted average grant date fair value per share (in dollars per share)	$ 250.09
Awards distributed (in shares)	(20)
Awards distributed, weighted average grant date fair value per share (in dollars per share)	$ 208.52
Nonvested (in shares)	51	37
Nonvested, weighted average grant date fair value per share (in dollars per share)	$ 252.86	$ 206.56

The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

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Weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect as a result of the occurrence of a terminating event.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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Per share or unit weighted-average fair value of nonvested award under share-based payment arrangement. Excludes share and unit options.

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Weighted average remaining contractual term for equity-based awards excluding options, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

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The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

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Intrinsic value of outstanding award under share-based payment arrangement. Excludes share and unit options.

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