Earnings Per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Earnings Per Share, Basic and Diluted
Net earnings	$ 113,029	$ 140,267	$ 97,136	$ 107,911	$ 115,010	$ 98,175	$ 88,162	$ 105,850	$ 458,343	$ 407,197	$ 391,240
Less: net earnings attributable to unvested restricted stock									7,497	6,638	6,982
Net earnings available to common stock									$ 450,846	$ 400,559	$ 384,258
Weighted-average common shares outstanding	95,373	97,909	98,442	98,662	98,770	98,723	98,870	99,056	97,591	98,853	100,176
Dilutive effect of outstanding securities (determined using treasury stock method)									260	260	280
Weighted-average common shares and potential common shares outstanding	95,636	98,134	98,660	98,913	99,046	98,993	99,121	99,287	97,851	99,113	100,456
Earnings Per Share, Basic	$ 1.17	$ 1.41	$ 0.97	$ 1.08	$ 1.15	$ 0.98	$ 0.88	$ 1.05	$ 4.62	$ 4.05	$ 3.84
Earnings Per Share, Diluted	$ 1.16	$ 1.41	$ 0.97	$ 1.07	$ 1.14	$ 0.98	$ 0.87	$ 1.05	$ 4.61	$ 4.04	$ 3.83