Note 6 - Stock-based Compensation - Restricted Stock Units (Details)	6 Months Ended
	Jun. 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	(93,856)
Vested, weighted-average grant date fair value (in dollars per share) | $ / shares	$ 41.87
Forfeited (in shares) | shares	(23,324)
Forfeited, weighted-average grant date fair value (in dollars per share) | $ / shares	$ 29.92
Balance (in shares) | shares	236,156
Balance, weighted-average grant date fair value (in dollars per share) | $ / shares	$ 32.52
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	0
Granted, weighted-average grant date fair value (in dollars per share) | $ / shares	$ 0
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	218,742
Balance, weighted-average grant date fair value (in dollars per share) | $ / shares	$ 28.77