Earnings Per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Earnings Per Share [Abstract]
Net earnings	$ 61,364	$ 81,016	$ 93,798	$ 114,914
Less: earnings allocable to participating securities	112	399	194	556
Net earnings available for common shareholders	$ 61,252	$ 80,617	$ 93,604	$ 114,358
Basic net earnings per share (in dollars per share)	$ 1.11	$ 1.47	$ 1.69	$ 2.08
Diluted net earnings per share (in dollars per share)	$ 1.10	$ 1.46	$ 1.68	$ 2.07
Basic weighted average shares outstanding (in shares)	55,302,000	54,942,000	55,245,000	54,851,000
Add: Dilutive effects of stock options and RSUs (in shares)	417,000	352,000	406,000	451,000
Dilutive weighted average shares outstanding (in shares)	55,719,000	55,294,000	55,651,000	55,302,000
Antidilutive securities excluded from computation of earnings per share, amount (in shares)	300,000	700,000	700,000	300,000