Guarantees and Long Term Standby Purchase Commitments - Future Minimum Lease Payments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Commitments and Contingencies Disclosure [Abstract]
Operating Leases, Future Minimum Payments Due, Next Twelve Months	$ 1,814
Operating Leases, Rent Expense, Net	1,600	$ 1,300	$ 1,300
Contractual Obligation, Due in Next Fiscal Year	1,869
Operating Leases, Future Minimum Payments, Due in Two Years	1,931
Contractual Obligation, Due in Second Year	835
Operating Leases, Future Minimum Payments, Due in Three Years	1,925
Contractual Obligation, Due in Third Year	676
Operating Leases, Future Minimum Payments, Due in Four Years	1,965
Contractual Obligation, Due in Fourth Year	193
Operating Leases, Future Minimum Payments, Due in Five Years	2,011
Contractual Obligation, Due in Fifth Year	0
Operating Leases, Future Minimum Payments, Due Thereafter	3,305
Contractual Obligation, Due after Fifth Year	0
Operating Leases, Future Minimum Payments Due	12,951
Contractual Obligation	$ 3,573