Reconciliation of Earnings Per Share (Details) - Earnings Per Share Reconciliation (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Earnings Per Share Reconciliation
Net Income, Basic EPS	$ 57,406	$ 94,268
Basic Shares Oustanding (in shares)	72,102	72,001
Earnings Per Share, Basic (dollars per share)	$ 0.80	$ 1.31
Effect of dilutive securities
Net Income, Diluted EPS	$ 57,406	$ 94,268
Diluted Shares Outstanding (in shares)	72,326	72,240
Earnings Per Share, Diluted (dollars per share)	$ 0.79	$ 1.30
Stock options
Effect of dilutive securities
Incremental Common Shares Attributable to Share-based Payment Arrangements	216	233
Non-vested restricted stock
Effect of dilutive securities
Incremental Common Shares Attributable to Share-based Payment Arrangements	8	6