Note 4 - Earnings Per Share - Basic and Diluted Loss Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Net income	$ 50,580	$ 54,593	$ 40,550	$ 53,392	$ 63,899	$ 38,764	$ 145,723	$ 156,055
Less: distributed earnings allocated to participating securities	(122)			(106)			(343)	(307)
Less: undistributed earnings allocated to participating securities	(202)			(222)			(554)	(613)
Net income attributable to common stockholders (numerator for basic earnings per share)	50,256			53,064			144,826	155,135
Add back: undistributed earnings allocated to participating securities	202			222			554	613
Less: undistributed earnings reallocated to participating securities	(195)			(218)			(540)	(602)
Numerator for diluted earnings per share under two class method	$ 50,263			$ 53,068			$ 144,840	$ 155,146
Weighted-average common shares outstanding (in shares)	61,978,195			60,665,349			61,422,925	60,505,916
Denominator for diluted earnings per share under two class method (in shares)	63,968,215			61,804,792			63,360,535	61,592,092
Basic Earnings Per Common Share (in dollars per share)	$ 0.81			$ 0.87			$ 2.36	$ 2.56
Diluted Earnings Per Common Share (in dollars per share)	$ 0.79			$ 0.86			$ 2.29	$ 2.52
Share-based Payment Arrangement, Option [Member]
Add: dilutive effect of share-based compensation (in shares)	1,990,020			1,139,443			1,570,194	1,086,176
Performance Shares [Member]
Add: dilutive effect of share-based compensation (in shares)	0			0			367,416	0