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
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net income	$ 320,886	$ 7,555	$ 334,177	$ 12,972
Weighted average shares outstanding - basic (in shares)	62,112,936	61,943,851	62,095,670	61,937,353
Dilutive securities outstanding (in shares)	497,118	368,373	552,482	382,675
Weighted average shares outstanding - diluted (in shares)	62,610,054	62,312,224	62,648,152	62,320,028
Earnings per share - basic (in dollars per share)	$ 5.17	$ 0.12	$ 5.38	$ 0.21
Earnings per share - diluted (in dollars per share)	$ 5.13	$ 0.12	$ 5.33	$ 0.21
Weighted Average antidilutive securities outstanding (in shares)	10,359	39,245	14,369	67,891
Stock options [Member]
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Weighted Average antidilutive securities outstanding (in shares)	9,875	16,493	12,533	17,093
Restricted shares [Member]
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Weighted Average antidilutive securities outstanding (in shares)	484	22,752	1,836	50,798