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 Reconciliation [Abstract]
Net earnings	$ 31,188	$ 28,514	$ 59,727	$ 59,449
Net earnings available to participating securities	(460)	0	(922)	0
Net earnings available to common shareholders	$ 30,728	$ 28,514	$ 58,805	$ 59,449
Weighted-average outstanding shares of common stock - basic	45,373,930	45,399,940	45,887,185	45,247,381
Dilutive effect of equity incentive plans	195,634	1,323,852	316,306	1,416,485
Weighted-average outstanding shares of common stock - diluted	45,569,564	46,723,792	46,203,491	46,663,866
Basic	$ 0.68	$ 0.63	$ 1.28	$ 1.31
Diluted	$ 0.67	$ 0.61	$ 1.27	$ 1.27
Share options excluded from the computation of diluted earnings per share	613,059	396,808	560,059	396,316