Note 5 - Earnings Per Share - Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Net income	$ 72,244	$ 73,664	$ 138,923	$ 137,480
Weighted-average shares:
Basic weighted-average number of common shares outstanding (in shares)	80,106,329	81,236,315	80,279,859	81,180,160
Diluted (in shares)	80,302,679	81,774,986	80,501,800	81,727,977
Average stock options and restricted stock units with anti-dilutive effect (in shares)	66,339	6,521	55,502	22,624
Basic (in dollars per share)	$ 0.90	$ 0.91	$ 1.73	$ 1.69
Diluted (in dollars per share)	$ 0.90	$ 0.90	$ 1.73	$ 1.68
Warrant [Member]
Weighted-average shares:
Dilutive effect of weighted-average outstanding common share equivalents (in shares)	0	278,080	0	286,718
Restricted Stock Units (RSUs) [Member]
Weighted-average shares:
Dilutive effect of weighted-average outstanding common share equivalents (in shares)	196,350	260,591	221,941	261,099