Note 6 - Share-based Compensation - Performance Share Units (Details) - Performance Shares [Member] - $ / shares	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Number of shares unvested at beginning of period (in shares)	375,528	360,512	328,187
Number of shares unvested at beginning of period, weighted average grant date fair value (in dollars per share)	$ 95.67	$ 93.74	$ 71.68
Number of shares granted (in shares)	202,023	142,156	150,763
Number of shares granted, weighted average grant date fair value (in dollars per share)	$ 112.87	$ 98.58	$ 122.57
Number of shares vested (in shares)	(145,038)	(127,140)	(118,438)
Number of shares vested, weighted average grant date fair value (in dollars per share)	$ 89.75	$ 93.46	$ 69.29
Number of shares forfeited (in shares)	(98,588)	0	0
Number of shares unvested at end of period, weighted average grant date fair value (in dollars per share)	$ 110.19	$ 0	$ 0
Number of shares unvested at end of period (in shares)	333,925	375,528	360,512
Number of shares unvested at end of period, weighted average grant date fair value (in dollars per share)	$ 109.57	$ 95.67	$ 93.74

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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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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