General Insurance Expenses (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Selling, General and Administrative Expense [Abstract]
Compensation	$ 646,595	$ 625,364	$ 564,008
Commissions	241,357	222,028	206,360
Other	309,661	333,835	308,628
Total general insurance expenses	$ 1,197,613	$ 1,181,227	$ 1,078,996