Earnings Per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Earnings Per Share [Abstract]
Net earnings	$ 18,126	$ 134,001	$ 133,040	$ 186,670
Less: earnings allocable to participating securities	118	1,207	709	1,797
Net earnings available for common shareholders	$ 18,008	$ 132,794	$ 132,331	$ 184,873
Basic net earnings per share (in dollars per share)	$ 0.33	$ 2.45	$ 2.41	$ 3.42
Diluted net earnings per share (in dollars per share)	$ 0.33	$ 2.42	$ 2.39	$ 3.38
Basic weighted average shares outstanding (in shares)	54,945,000	54,211,000	54,883,000	54,029,000
Add: Dilutive effects of stock options and RSUs (in shares)	336,000	654,000	395,000	743,000
Dilutive weighted average shares outstanding (in shares)	55,281,000	54,865,000	55,278,000	54,772,000
Antidilutive securities excluded from computation of earnings per share, amount (in shares)	700,000	0	600,000	0