Commitments and Contingencies - Operating Leases - Additional Information (Detail) $ in Millions				1 Months Ended		12 Months Ended
	Jan. 01, 2018 ft²	Aug. 15, 2017 ft²	May 31, 2017 ft²	May 31, 2017 ft²	May 31, 2016 ft²	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Commitments And Contingencies [Line Items]
Rent expense, net of sublease income | $						$ 3.7	$ 2.0	$ 0.4
Operating Lease One [Member]
Commitments And Contingencies [Line Items]
Office space rent under operating lease				22,067	19,805
Lease expire date				Feb. 28, 2022	Feb. 28, 2022
Lease existence of option to extend			true
Commencement date of lease					Sep. 01, 2016
Sixth Amendment to Lease [Member]
Commitments And Contingencies [Line Items]
Lease expire date			Aug. 15, 2024
Increased office space rent		8,200	32,876
Sixth Amendment to Lease [Member] | Subsequent Event [Member]
Commitments And Contingencies [Line Items]
Increased office space rent	24,676