Earnings Per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Earnings Per Share [Abstract]
Net earnings	$ 81,016	$ 105,166	$ 114,914	$ 52,669
Less: earnings allocated to participating securities	399	1,012	556	544
Earnings available for common shareholders	$ 80,617	$ 104,154	$ 114,358	$ 52,125
Basic net earnings per share (in dollars per share)	$ 1.47	$ 1.93	$ 2.08	$ 0.97
Diluted net earnings per share (in dollars per share)	$ 1.46	$ 1.90	$ 2.07	$ 0.95
Weighted average shares outstanding-basic (in shares)	54,942,000	54,004,000	54,851,000	53,936,000
Add: Dilutive effects of stock options and RSUs (in shares)	352,000	704,000	451,000	755,000
Weighted average shares outstanding-dilutive (in shares)	55,294,000	54,708,000	55,302,000	54,691,000
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount (in shares)	700,000		300,000