Commitments and Contingencies - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Disclosure Commitments And Contingencies Additional Information Details [Line Items]
Rent expense	$ 4.5	$ 3.3	$ 1.4
Novato Office Facility
Disclosure Commitments And Contingencies Additional Information Details [Line Items]
Operating lease expiration year	2026
Lease renewal term	5 years