Note 10 - Stock Based Compensation - Summary of Stock Option Activity (Details) - Employee Stock Option [Member]	3 Months Ended
	Mar. 31, 2018 $ / shares shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Outstanding, beginning balance (in shares) | shares	1,484,914
Exercised (in shares) | shares	(112,669)
Forfeited or expired (in shares) | shares	(5,311)
Outstanding, ending balance (in shares) | shares	1,366,934
Exercisable (in shares) | shares	636,486
Outstanding, beginning balance, weighted average exercise price (in dollars per share) | $ / shares	$ 32.53
Exercised, weighted average exercise price (in dollars per share) | $ / shares	24.16
Forfeited or expired (in dollars per share) | $ / shares	36.94
Outstanding, Ending balance, weighted average exercise price (in dollars per share) | $ / shares	33.20
Exercisable, weighted average exercise price (in dollars per share) | $ / shares	$ 25.41