Nature of Business and Significant Accounting Policies (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Numerator for earnings per share:
Earnings from continuing operations	$ 200,261	$ 177,015	$ 225,356
Loss from discontinued operations, net of tax	0	0	(2,958)
Net earnings for basic and diluted earnings per share	$ 200,261	$ 177,015	$ 222,398
Denominator for earnings per share:
Weighted average number of common shares outstanding	141,143	142,446	151,634
Assumed exercise of stock options and vesting of restricted stock awards, net of shares assumed reacquired under the treasury stock method	2,426	2,149	1,960
Diluted shares outstanding	143,569	144,595	153,594