Note 4 - Earnings Per Common Share - Basic and Diluted Earnings Per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Net income	$ 93,352	$ 132,348	$ 274,966	$ 317,769
Less: Net income allocated to unvested restricted stock	(400)	(294)	(1,104)	(635)
Net income available to common stockholders for basic earnings per common share	92,952	132,054	273,862	317,134
Effect of dilutive securities:
Net income available to common stock for diluted earnings per share	$ 93,179	$ 132,289	$ 276,753	$ 319,950
Weighted average basic common shares outstanding (in shares)	110,205,460	118,338,891	112,778,362	119,188,145
Share-based awards (in shares)	875,214	643,602	881,500	605,860
Weighted average diluted shares outstanding (in shares)	124,449,912	136,077,415	129,521,479	136,888,927
Income Per Common Share:
Basic (in dollars per share)	$ 0.84	$ 1.12	$ 2.43	$ 2.66
Diluted (in dollars per share)	$ 0.75	$ 0.97	$ 2.14	$ 2.34
1.625% Convertible Senior Notes due May 2018 [Member]
Effect of dilutive securities:
Interest on convertible senior notes	$ 94	$ 91	$ 1,123	$ 1,088
Interest on convertible senior notes (in shares)	7,174,013	7,165,845	7,174,013	7,165,845
0.25% Convertible Senior Notes Due June 2019 [Member]
Effect of dilutive securities:
Interest on convertible senior notes	$ 85	$ 82	$ 1,016	$ 984
Interest on convertible senior notes (in shares)	3,642,200	3,638,080	3,642,200	3,638,080
1.25% Convertible Senior Notes Due August 2032 [Member]
Effect of dilutive securities:
Interest on convertible senior notes	$ 48	$ 62	$ 752	$ 744
Interest on convertible senior notes (in shares)	2,553,025	6,290,997	5,045,404	6,290,997