Commitments and Contingencies - Lease Financing Obligation (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Lease Financing Obligation, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2018	$ 6,113
2019	6,293
2020	6,477
2021	6,674
2022	6,871
Thereafter	5,264
Total payments	37,692
Less: interest and land lease expense	(21,730)
Total payments under facility financing obligations	15,962
Property reverting to landlord	23,630
Present value of obligation	39,592
Less: current portion	(1,919)
Long-term portion of obligation	$ 37,673	$ 39,593