Note 10 - Stock-based Compensation - Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Beginning Balance, Shares (in shares)	4,402,000
Beginning Balance, Weighted Average Exercise Price (in dollars per share)	$ 17.07
Granted, Shares (in shares)	560,000
Granted, Weighted Average Exercise Price (in dollars per share)	$ 39.52
Exercised, Shares (in shares)	(1,443,685)	(9,043)
Exercised, Weighted Average Exercise Price (in dollars per share)	$ 16.03
Forfeitures, Shares (in shares)	(134,000)
Forfeitures, Weighted Average Exercise Price (in dollars per share)	$ 18.99
Expired, Shares (in shares)
Expired, Weighted Average Exercise Price (in dollars per share)
Ending Balance, Shares (in shares)	3,385,000
Ending Balance, Weighted Average Exercise Price (in dollars per share)	$ 21.19
Ending Balance, Weighted Average Remaining Contractual Terms (Year)	6 years 310 days
Outstanding, Aggregate intrinsic value	$ 33,767
Expected to Vest, Shares (in shares)	1,061,000
Expected to Vest, Weighted Average Exercise Price (in dollars per share)	$ 27.50
Expected to Vest, Weighted Average Remaining Contractual Terms (Year)	9 years 51 days
Expected to Vest, Aggregate Intrinsic Value	$ 6,486
Exercisable, Shares (in shares)	1,899,000
Exercisable, Weighted Average Exercise Price (in dollars per share)	$ 16.67
Exercisable, Weighted Average Remaining Contractual Terms (Year)	5 years 51 days
Exercisable, Aggregate Intrinsic Value	$ 24,582