Earnings Per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Earnings Per Share [Abstract]
Net earnings	$ 32,434	$ 33,898
Less: earnings allocable to participating securities	81	171
Net earnings available for common shareholders	$ 32,353	$ 33,727
Basic net earnings per share (in dollars per share)	$ 0.59	$ 0.62
Diluted net earnings per share (in dollars per share)	$ 0.58	$ 0.61
Basic weighted average shares outstanding (in shares)	55,186,000	54,759,000
Add: Dilutive effects of stock options and RSUs (in shares)	379,000	504,000
Dilutive weighted average shares outstanding (in shares)	55,565,000	55,263,000
Antidilutive securities excluded from computation of earnings per share, amount (in shares)	800,000	300,000