Reconciliation of Earnings Per Share (EPS) - Earnings Per Share Reconciliation (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Earnings Per Share Reconciliation
Net Income, Basic EPS	$ 83,067	$ 131,287	$ 139,759	$ 188,693
Basic Shares Oustanding (in shares)	72,167	72,117	72,155	72,110
Earnings Per Share, Basic (in dollars per share)	$ 1.15	$ 1.82	$ 1.94	$ 2.62
Effect of dilutive securities
Net Income, Diluted EPS	$ 83,067	$ 131,287	$ 139,759	$ 188,693
Diluted Shares Outstanding (in shares)	72,419	72,330	72,329	72,336
Earnings Per Share, Diluted (dollars per share)	$ 1.15	$ 1.82	$ 1.93	$ 2.61
Stock options
Effect of dilutive securities
Incremental common shares attributable to share-based payment arrangements	225	205	164	218
Non-vested restricted stock
Effect of dilutive securities
Incremental common shares attributable to share-based payment arrangements	16	8	10	8
Performance share awards
Effect of dilutive securities
Incremental common shares attributable to share-based payment arrangements	0	0