Commitments and Contingencies (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Commitments and Contingencies Disclosure [Abstract]
Operating Leases, Rent Expense	$ 51,000	$ 49,300	$ 46,000
Operating Leases, Future Minimum Payments Due, Next Twelve Months	42,970
Operating Leases, Future Minimum Payments, Due in Two Years	39,005
Operating Leases, Future Minimum Payments, Due in Three Years	34,236
Operating Leases, Future Minimum Payments, Due in Four Years	27,715
Operating Leases, Future Minimum Payments, Due in Five Years	21,996
Operating Leases, Future Minimum Payments, Due Thereafter	44,496
Operating Leases, Future Minimum Payments Due	$ 210,418	$ 213,200