Reconciliation of Earnings Per Share (Details) - Earnings Per Share Reconciliation (USD $) In Thousands, except Per 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 Reconciliation
Net Income, Basic EPS	$ 131,287	$ 63,325	$ 188,693	$ 157,593
Basic Shares Oustanding (in shares)	72,117	72,065	72,110	72,033
Earnings Per Share, Basic (dollars per share)	$ 1.82	$ 0.88	$ 2.62	$ 2.19
Effect of dilutive securities
Net Income, Diluted EPS	$ 131,287	$ 63,325	$ 188,693	$ 157,593
Diluted Shares Outstanding (in shares)	72,330	72,420	72,336	72,364
Earnings Per Share, Diluted (dollars per share)	$ 1.82	$ 0.87	$ 2.61	$ 2.18
Stock options
Effect of dilutive securities
Incremental Common Shares Attributable to Share-based Payment Arrangements	205	347	218	324
Non-vested restricted stock
Effect of dilutive securities
Incremental Common Shares Attributable to Share-based Payment Arrangements	8	8	8	7