Nature of Business and Significant Accounting Policies (Details) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Nature of Business and Significant Accounting Policies [Abstract]
Cost Method Investments	$ 5,200	$ 5,200	$ 13,500
Numerator for earnings per share:
Net earnings for basic and diluted earnings per share		$ 241,686	$ 200,261	$ 177,015
Denominator for earnings per share:
Weighted average number of common shares outstanding		139,353	141,143	142,446
Assumed exercise of stock options and vesting of restricted stock awards, net of shares assumed reacquired under the treasury stock method		1,421	2,426	2,149
Diluted shares outstanding		140,774	143,569	144,595
Cost Method Investments, Original Cost	8,300	$ 8,300
Cost method investment, sale price	28,500
Cost-method Investments, Realized Gains	20,200
cost method investments, cash received	25,600
Cost method investments, receivable from sale	$ 2,900	$ 2,900