Note 4 - Earnings Per Common Share - Components Used in Computation of Basic and Diluted Earnings Per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Net income	$ 82,620	$ 72,661
Less: Net income allocated to unvested restricted stock	(301)	(113)
Net income available to common stockholders for basic earnings per common share	82,319	72,548
Effect of dilutive securities:
Net income available to common stock for diluted earnings per share	$ 82,562	$ 72,322
Basic (in shares)	114,487,245	120,814,939
Share-based awards (in shares)	903,173	561,065
Weighted average diluted shares outstanding (in shares)	132,505,435	138,430,580
Income Per Common Share:
Basic (in dollars per share)	$ 0.72	$ 0.60
Diluted (in dollars per share)	$ 0.62	$ 0.52
1.625% Convertible Senior Notes due May 2018 [Member]
Effect of dilutive securities:
Interest on convertible senior notes	$ 94	$ (370)
Interest on convertible senior notes (in shares)	7,169,940	7,157,934
0.25% Convertible Senior Notes Due June 2019 [Member]
Effect of dilutive securities:
Interest on convertible senior notes	$ 85	$ 82
Interest on convertible senior notes (in shares)	3,640,140	3,634,072
1.25% Convertible Senior Notes Due August 2032 [Member]
Effect of dilutive securities:
Interest on convertible senior notes	$ 64	$ 62
Interest on convertible senior notes (in shares)	6,304,937	6,262,570