Note 6 - Stock-based Compensation - Restricted Stock Units (Details)	9 Months Ended
	Sep. 30, 2021 $ / shares shares
Time-based Restricted Stock Units [Member]
Balance (in shares) | shares	292,670
Balance, weighted-average grant date fair value (in dollars per share) | $ / shares	$ 33.37
Granted (in shares) | shares	60,666
Granted, weighted-average grant date fair value (in dollars per share) | $ / shares	$ 41.11
Vested (in shares) | shares	(96,869)
Vested, weighted-average grant date fair value (in dollars per share) | $ / shares	$ 41.72
Forfeited (in shares) | shares	(23,324)
Forfeited, weighted-average grant date fair value (in dollars per share) | $ / shares	$ 29.92
Balance (in shares) | shares	233,143
Balance, weighted-average grant date fair value (in dollars per share) | $ / shares	$ 32.26
Performance-based Restricted Stock Units [Member]
Balance (in shares) | shares	302,802
Balance, weighted-average grant date fair value (in dollars per share) | $ / shares	$ 32.55
Granted (in shares) | shares	113,764
Granted, weighted-average grant date fair value (in dollars per share) | $ / shares	$ 37.13
Vested (in shares) | shares	(76,292)
Vested, weighted-average grant date fair value (in dollars per share) | $ / shares	$ 42.56
Forfeited (in shares) | shares	(7,768)
Forfeited, weighted-average grant date fair value (in dollars per share) | $ / shares	$ 40.60
Balance (in shares) | shares	332,506
Balance, weighted-average grant date fair value (in dollars per share) | $ / shares	$ 31.63