Earnings Per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Basic net income per share:
Net income	$ 32,024	$ 44,294	$ 65,110	$ 114,771
Weighted average number of common shares outstanding	47,312,154	48,419,556	47,186,248	48,342,756
Basic net income per share	$ 0.68	$ 0.91	$ 1.38	$ 2.37
Diluted net income per share:
Net income	$ 32,024	$ 44,294	$ 65,110	$ 114,771
Weighted average number of common shares outstanding	47,312,154	48,419,556	47,186,248	48,342,756
Dilutive effect of stock options and non-vested stock awards	1,245,492	1,764,664	1,329,421	1,813,532
Dilutive effect of assumed conversion of the 2.0% Convertible Senior Notes	967,141	2,196,048	1,578,640	1,818,377
Dilutive effect of warrants	914,700	1,161,100	1,076,618	683,638
Weighted average number of common shares and dilutive potential common shares outstanding	50,439,487	53,541,368	51,170,927	52,658,303
Diluted net income per share	$ 0.63	$ 0.83	$ 1.27	$ 2.18
Earnings Per Share (Textual) [Abstract]
Antidilutive common stock shares	86,760	4,835	59,554	2,431