Reconciliation of Earnings Per Share (Reconciliation) (Details) (USD $) In Thousands, except 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 Reconciliation
Net Income, Basic EPS	$ 62,823	$ 2,046	$ 131,287	$ 57,406	$ 14,432	$ 87,599	$ 63,325	$ 94,268	$ 253,562	$ 259,624	$ 290,807
Basic Shares Oustanding (in shares)									72,119,021	72,055,661	71,845,463
Earnings Per Share, Basic (dollars per share)	$ 0.87	$ 0.03	$ 1.82	$ 0.80	$ 0.20	$ 1.22	$ 0.88	$ 1.31	$ 3.52	$ 3.60	$ 4.05
Effect of dilutive securities
Net Income, Diluted EPS	$ 62,823	$ 2,046	$ 131,287	$ 57,406	$ 14,432	$ 87,599	$ 63,325	$ 94,268	$ 253,562	$ 259,624	$ 290,807
Diluted Shares Outstanding (in shares)									72,316,214	72,332,369	72,050,997
Earnings Per Share, Diluted (dollars per share)	$ 0.87	$ 0.03	$ 1.82	$ 0.79	$ 0.20	$ 1.21	$ 0.87	$ 1.30	$ 3.51	$ 3.59	$ 4.04
Stock options
Effect of dilutive securities
Incremental Common Shares Attributable to Share-based Payment Arrangements									196,000	270,000	190,000
Non-vested restricted stock
Effect of dilutive securities
Incremental Common Shares Attributable to Share-based Payment Arrangements									1,000	6,000	16,000