Note 6 - Stock-based Compensation - Restricted Stock Units (Details)	3 Months Ended
	Mar. 31, 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	13,509
Granted, weighted-average grant date fair value (in dollars per share) | $ / shares	$ 39.77
Vested (in shares) | shares	(91,654)
Vested, weighted-average grant date fair value (in dollars per share) | $ / shares	$ 41.88
Forfeited (in shares) | shares	(16,296)
Forfeited, weighted-average grant date fair value (in dollars per share) | $ / shares	$ 30.62
Balance (in shares) | shares	198,229
Balance, weighted-average grant date fair value (in dollars per share) | $ / shares	$ 30.10
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	(8,537)
Vested, weighted-average grant date fair value (in dollars per share) | $ / shares	$ 43.37
Forfeited (in shares) | shares	(5,008)
Forfeited, weighted-average grant date fair value (in dollars per share) | $ / shares	$ 40.60
Balance (in shares) | shares	289,257
Balance, weighted-average grant date fair value (in dollars per share) | $ / shares	$ 32.09