Commitments and Contingencies - Operating Leases (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	$ 6,400	$ 5,600	$ 5,900
Future Operating Lease Minimum Payments, Due Next Year	5,345
Future Operating Lease Minimum Payments, Due in Two Years	4,494
Future Operating Lease Minimum Payments, Due in Three Years	3,708
Future Operating Lease Minimum Payments, Due in Four Years	3,208
Future Operating Lease Minimum Payments, Due in Five Years	2,812
Future Operating Lease Minimum Payments, Due Therafter	$ 8,033