Commitments and Contingencies - Future Minimum Lease Payments Under Operating and Capital Leases (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Commitments And Contingencies Disclosure [Abstract]
Operating leases, 2018	$ 4,218
Operating leases, 2019	4,252
Operating leases, 2020	4,379
Operating leases, 2021	4,511
Operating leases, 2022	4,646
Operating leases, Thereafter	13,374
Operating leases, Total minimum lease payments	35,380
Capital leases, 2018	1,024
Capital leases, 2019	574
Capital leases, 2020	269
Capital leases, 2021	2
Capital leases, Total minimum lease payments	1,869
Less: Portion representing interest	(188)
Present value of capital lease obligations	1,681
Less: Current portion of capital lease obligations	(936)	$ (1,130)
Capital lease obligations, net of current portion	$ 745	$ 1,698