Nature of Business and Significant Accounting Policies (Details) - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator for earnings per share:
Net earnings for basic and diluted earnings per share	$ 107,223	$ 166,626	$ 241,686
Denominator for earnings per share:
Weighted average number of common shares outstanding	137,456	137,138	139,353
Assumed exercise of stock options and vesting of restricted stock awards, net of shares assumed reacquired under the treasury stock method	2,190	1,359	1,421
Diluted shares outstanding	139,646	138,497	140,774