Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Earnings Per Share, Basic and Diluted
Net earnings	$ 80,417	$ 97,136	$ 193,334	$ 205,047
Less: net earnings attributable to unvested restricted stock	(1,207)	(1,590)	(2,848)	(3,282)
Net earnings available to common stock	$ 79,210	$ 95,546	$ 190,486	$ 201,765
Weighted-average common shares outstanding	94,165	98,442	94,522	98,551
Dilutive effect of outstanding securities (determined using treasury stock method)	293	218	276	240
Weighted-average common shares and potential common shares outstanding	94,458	98,660	94,798	98,791
Earnings Per Share, Basic	$ 0.84	$ 0.97	$ 2.02	$ 2.05
Earnings Per Share, Diluted	$ 0.84	$ 0.97	$ 2.01	$ 2.04
Net earnings available to common stock	$ 79,210	$ 95,546	$ 190,486	$ 201,765