Earnings Per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended										12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012		Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011		Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Earnings Per Share [Line Items]
Income available to common stockholders, Basic											$ 116,911	$ 191,164	$ 152,198
Effect of dilutive securities: Options											0	0	0
Income available to common stockholders after assumed conversions, Diluted											$ 116,911	$ 191,164	$ 152,198
Weighted Shares, Basic											54,899	54,825	54,792
Effect of dilutive securities: Options											23	20	34
Weighted Shares, Diluted											54,922	54,845	54,826
Per-Share Amount, Basic	$ (0.32)	$ 1.21	$ (0.10)		$ 1.34	$ 1.45	$ (0.07)		$ 1.04	$ 1.06	$ 2.13	$ 3.49	$ 2.78
Per-Share Amount, Diluted	$ (0.32)	$ 1.21	$ (0.10)	[1]	$ 1.34	$ 1.45	$ (0.07)	[1]	$ 1.04	$ 1.06	$ 2.13	$ 3.49	$ 2.78
Potentially dilutive securities (in shares)											415	504	448
Common Stock [Member]
Earnings Per Share [Line Items]
Potentially dilutive securities (in shares)											80	103	93

[1] The dilutive impact of incremental shares is excluded from loss position in accordance with GAAP.