Earnings Per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Basic net income per share:
Net income	$ 32,776	$ 48,074	$ 44,294	$ 70,477	$ 79,628	$ 84,348	$ 67,213	$ 42,182	$ 195,621	$ 273,371	$ 149,279
Weighted average number of common shares outstanding									47,731,265	49,522,202	51,647,267
Basic net income per share									$ 4.10	$ 5.52	$ 2.89
Diluted net income per share:
Net income	$ 32,776	$ 48,074	$ 44,294	$ 70,477	$ 79,628	$ 84,348	$ 67,213	$ 42,182	$ 195,621	$ 273,371	$ 149,279
Weighted average number of common shares outstanding									47,731,265	49,522,202	51,647,267
Dilutive effect of stock options and non-vested stock awards									1,593,018	1,085,806	662,001
Dilutive effect of assumed conversion of the 2.0% Convertible Senior Notes									1,525,271
Dilutive effect of warrants									313,554
Weighted average number of common shares and dilutive potential common shares outstanding	49,179,891	50,253,757	53,541,368	51,534,794	49,924,608	50,197,740	51,318,044	51,226,435	51,163,108	50,608,008	52,309,268
Diluted net income per share	$ 0.67	$ 0.96	$ 0.83	$ 1.37	$ 1.59	$ 1.68	$ 1.31	$ 0.82	$ 3.82	$ 5.40	$ 2.85
Earnings Per Share (Textual) [Abstract]
Antidilutive common stock shares									50,857	895,899	2,676,447
Interest rate of convertible senior notes	2.00%				2.00%				2.00%	2.00%	2.00%