Earnings Per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Earnings Per Share, Basic and Diluted
Net earnings	$ 107,911	$ 105,850
Less: net earnings attributable to unvested restricted stock	(1,692)	(1,782)
Net earnings available to common stock	$ 106,219	$ 104,068
Weighted-average common shares outstanding	98,662	99,056
Dilutive effect of outstanding securities (determined using treasury stock method)	251	231
Weighted-average common shares and potential common shares outstanding	98,913	99,287
Earnings Per Share, Basic	$ 1.08	$ 1.05
Earnings Per Share, Diluted	$ 1.07	$ 1.05