Reconciliation of Earnings Per Share (EPS) - Earnings Per Share Reconciliation (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Earnings Per Share Reconciliation
Net Income, Basic EPS	$ 56,692	$ 57,406
Basic Shares Oustanding (in shares)	72,143	72,102
Earnings Per Share, Basic (in dollars per share)	$ 0.79	$ 0.80
Effect of dilutive securities
Net Income, Diluted EPS	$ 56,692	$ 57,406
Diluted Shares Outstanding (in shares)	72,288	72,326
Earnings Per Share, Diluted (dollars per share)	$ 0.78	$ 0.79
Stock options
Effect of dilutive securities
Incremental common shares attributable to share-based payment arrangements	144	216
Non-vested restricted stock
Effect of dilutive securities
Incremental common shares attributable to share-based payment arrangements	1	8