Commitments and Contingencies - Operating Leases - Additional Information (Detail) $ in Millions				1 Months Ended				3 Months Ended	12 Months Ended
	Oct. 02, 2018 ft²	Sep. 30, 2018 ft²	Aug. 01, 2018 ft²	Dec. 31, 2018 ft²	Oct. 31, 2018 ft²	May 31, 2018 ft²	Apr. 30, 2018 ft²	Mar. 31, 2018 ft²	Dec. 31, 2018 USD ($) ft²	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Commitments And Contingencies [Line Items]
Rent expense | $									$ 6.5	$ 3.7	$ 2.0
Operating Lease One [Member]
Commitments And Contingencies [Line Items]
Office space rent under operating lease					58,442				58,442
Lease expire date					Aug. 31, 2024				Aug. 31, 2024
Increased office space rent		54,943			3,499
Operating Lease Two [Member]
Commitments And Contingencies [Line Items]
Office space rent under operating lease							40,419	19,805
Lease expire date							Aug. 31, 2024	Feb. 28, 2022	Aug. 31, 2024
Increased office space rent	7,133		13,481				20,614
Operating Lease Three [Member]
Commitments And Contingencies [Line Items]
Office space rent under operating lease						15,525
Lease expire date						Nov. 30, 2024
Operating Lease Five [Member]
Commitments And Contingencies [Line Items]
Office space rent under operating lease				15,975
Lease expire date				Feb. 28, 2024