Leases - Addtional Information (Detail) - San Francisco, California [Member] $ in Millions	1 Months Ended	9 Months Ended
	Oct. 31, 2018 USD ($) ft²	Sep. 30, 2018 USD ($)
Lessor Lease Description [Line Items]
Lease expiration date	Aug. 31, 2031	Feb. 28, 2027
Total rental payments to be received	$ 143.0	$ 167.3
Building occupancy rate by tenant		43.00%
Current lease incentive obligation to pay for tenant improvements	$ 2.4	$ 24.9
Area of Office Space Leased | ft²	117,560
Lease commencement date	Apr. 30, 2019