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Note 14 - Stock Option Plans - Stock Option Transactions (Details) - $ / shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Outstanding, Number of shares (in shares)	754,497	702,337	653,571
Outstanding, Weighted average exercise price per share (in dollars per share)	$ 49.94	$ 43.80	$ 36.69
Granted, Number of shares (in shares)	239,325	180,800	163,366
Granted, Weighted average exercise price per share (in dollars per share)	$ 61.21	$ 60.56	$ 62.80
Exercised, Number of shares (in shares)	(113,132)	(80,319)	(63,886)
Exercised, Weighted average exercise price per share (in dollars per share)	$ 30.53	$ 16.90	$ 12.86
Forfeited, Number of shares (in shares)	(26,468)	(31,986)	(50,714)
Forfeited, Weighted average exercise price per share (in dollars per share)	$ 60.29	$ 59.35	$ 52.33
Expired, Number of shares (in shares)	(9,202)	(16,335)	0
Expired, Weighted average exercise price per share (in dollars per share)	$ 58.96	$ 47.51	$ 0
Outstanding, Number of shares (in shares)	845,020	754,497	702,337
Outstanding, Weighted average exercise price per share (in dollars per share)	$ 55.31	$ 49.94	$ 43.80
Options exercisable, Number of shares (in shares)	401,345	356,066	326,937
Options exercisable, Weighted average exercise price per share (in dollars per share)	$ 49.21	$ 40.64	$ 28.91

X

- Definition

The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(iii)
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- Definition

The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan.

+ References

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-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
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- Definition

Number of options or other stock instruments for which the right to exercise has lapsed under the terms of the plan agreements.

+ References

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-Publisher FASB
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- Definition

The number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan.

+ References

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-Publisher FASB
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-Topic 718
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X

- Definition

Gross number of share options (or share units) granted during the period.

+ References

No definition available.

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Number of options outstanding, including both vested and non-vested options.

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Weighted average price at which option holders acquired shares when converting their stock options into shares.

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Weighted average price at which grantees could have acquired the underlying shares with respect to stock options of the plan that expired.

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Weighted average per share amount at which grantees can acquire shares of common stock by exercise of options.

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