Note 6 - Stock-based Compensation - Restricted Stock Units (Details)	9 Months Ended
	Sep. 30, 2020 $ / shares shares
Time-based Restricted Stock Units [Member]
Balance (in shares) | shares	273,200
Balance (in dollars per share) | $ / shares	$ 35.90
Granted (in shares) | shares	3,013
Granted (in dollars per share) | $ / shares	$ 24.89
Vested (in shares) | shares	(79,630)
Vested (in dollars per share) | $ / shares	$ 25.39
Forfeited (in shares) | shares	(8,162)
Forfeited (in dollars per share) | $ / shares	$ 39.41
Balance (in shares) | shares	188,421
Balance (in dollars per share) | $ / shares	$ 40.01
Performance-based Restricted Stock Units [Member]
Balance (in shares) | shares	297,744
Balance (in dollars per share) | $ / shares	$ 32.65
Granted (in shares) | shares	212,369
Granted (in dollars per share) | $ / shares	$ 19.66
Vested (in shares) | shares	(193,240)
Vested (in dollars per share) | $ / shares	$ 21.68
Forfeited (in shares) | shares	(14,071)
Forfeited (in dollars per share) | $ / shares	$ 39.08
Balance (in shares) | shares	302,802
Balance (in dollars per share) | $ / shares	$ 30.24