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		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Net income	$ 98,994	$ 112,760	$ 181,614	$ 185,421
Less: Net income allocated to unvested restricted stock	(408)	(251)	(705)	(350)
Net income available to common stockholders for basic earnings per common share	98,586	112,509	180,909	185,071
Net income available to common stock for diluted earnings per share	$ 98,829	$ 112,744	$ 182,120	$ 186,244
Basic (in shares)	113,689,435	118,419,937	114,086,136	119,617,438
Effect of dilutive securities:
Share-based awards (in shares)	821,628	583,264	868,491	575,516
Weighted average diluted shares outstanding (in shares)	131,636,412	136,088,146	132,079,976	137,277,899
Income Per Common Share:
Basic (in dollars per share)	$ 0.87	$ 0.95	$ 1.59	$ 1.55
Diluted (in dollars per share)	$ 0.75	$ 0.83	$ 1.38	$ 1.36
1.625% Convertible Senior Notes due May 2018 [Member]
Interest on convertible senior notes	$ 94	$ 91	$ 468	$ 453
Effect of dilutive securities:
Interest on convertible senior notes (in shares)	7,171,943	7,163,865	7,171,943	7,163,865
0.25% Convertible Senior Notes Due June 2019 [Member]
Interest on convertible senior notes	$ 85	$ 82	$ 423	$ 410
Effect of dilutive securities:
Interest on convertible senior notes (in shares)	3,641,157	3,637,091	3,641,157	3,637,091
1.25% Convertible Senior Notes Due August 2032 [Member]
Interest on convertible senior notes	$ 64	$ 62	$ 320	$ 310
Effect of dilutive securities:
Interest on convertible senior notes (in shares)	6,312,249	6,283,989	6,312,249	6,283,989