Commitments and Contingencies - Additional Information (Detail) $ in Thousands	1 Months Ended	9 Months Ended
	Nov. 30, 2017 USD ($) ft² Option	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Commitments And Contingencies [Line Items]
Restricted cash		$ 79,941	$ 3,500
Brisbane Build-to-Suit Lease [Member]
Commitments And Contingencies [Line Items]
Property leased | ft²	87,700
Lease expiration date		May 31, 2029
Operating lease expiration period	10 years
Number of lease term extension options | Option	2
Restricted cash	$ 3,500	$ 3,500
Costs capitalized		$ 39,500
Brisbane Build-to-Suit Lease [Member] | Building [Member]
Commitments And Contingencies [Line Items]
Costs capitalized	$ 20,900
Brisbane Build-to-Suit Lease [Member] | Maximum [Member]
Commitments And Contingencies [Line Items]
Lease agreement, extendable lease term	10 years