Note 5 - Share-based Compensation - Restricted Share Units (Details) - Restricted Stock Units (RSUs) [Member] - $ / shares	12 Months Ended
Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Number of shares unvested at beginning of period (in shares)	1,141,337	1,290,232	1,345,146
Number of shares unvested at beginning of period, weighted average grant date fair value (in dollars per share)	$ 129.75	$ 110.83	$ 97.22
Number of shares granted (in shares)	277,234	317,751	360,734
Number of shares granted, weighted average grant date fair value (in dollars per share)	$ 180	$ 189.66	$ 150.33
Number of shares vested (in shares)	(443,883)	(427,942)	(387,948)
Number of shares vested, weighted average grant date fair value (in dollars per share)	$ 125.36	$ 118	$ 100.36
Number of shares forfeited (in shares)	(38,196)	(38,704)	(27,700)
Number of shares forfeited, weighted average grant date fair value (in dollars per share)	$ 143.45	$ 138.94	$ 118.2
Number of shares unvested at end of period (in shares)	936,492	1,141,337	1,290,232
Number of shares unvested at end of period, weighted average grant date fair value (in dollars per share)	$ 147.02	$ 129.75	$ 110.83

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

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- Definition

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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- Definition

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.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 718
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(i)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480429/718-10-50-2

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 718
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(ii)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480429/718-10-50-2

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- Definition

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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- Definition

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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