Leases - Addtional Information (Detail) $ in Thousands	3 Months Ended	12 Months Ended
	Jun. 30, 2017 USD ($) ft²	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Lessor Lease Description [Line Items]
Total rental payments to be received		$ 166,413
Rent expense on operating leases		$ 5,700	$ 5,300	$ 4,500
San Francisco, California [Member]
Lessor Lease Description [Line Items]
Area of Office Space Leased | ft²	287,000
Lease expiration date	Feb. 28, 2027
Total rental payments to be received	$ 167,300
Building occupancy rate by tenant		43.00%
Incentive Receivable and obligation to pay for tenant improvements		$ 24,900
Accrued deferred lease origination costs	$ 6,500