Restructuring - Summary of Total Amount Expected to Be Incurred and Liability (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Restructuring Plan, July 2018 [Member]
Restructuring Cost and Reserve [Line Items]
Amounts accrued, Beginning balance	$ 3,634
Charges for the year		$ 9,155
Revised estimates during the year/ period	(1,942)	235
Total restructuring costs expected to be incurred		9,390
Amounts paid during the year/ period	(1,657)	(5,756)
Amounts accrued, Ending balance	35	3,634
Clinical Trial Closing Costs [Member] | Restructuring Plan, July 2018 [Member]
Restructuring Cost and Reserve [Line Items]
Amounts accrued, Beginning balance	3,540
Charges for the year		5,703
Revised estimates during the year/ period	(1,954)	41
Total restructuring costs expected to be incurred		5,744
Amounts paid during the year/ period	(1,579)	(2,204)
Amounts accrued, Ending balance	7	3,540
One-time Employee Termination Benefits [Member] | Restructuring Plan, July 2018 [Member]
Restructuring Cost and Reserve [Line Items]
Charges for the year		1,879
Revised estimates during the year/ period		2
Total restructuring costs expected to be incurred		1,881
Amounts paid during the year/ period		(1,881)
One-time Employee Termination Benefits [Member] | Restructuring Plan, November 2018 [Member]
Restructuring Cost and Reserve [Line Items]
Amounts accrued, Beginning balance	62
Total restructuring costs expected to be incurred	660	984
Amounts paid during the year/ period	(702)	(922)
Amounts accrued, Ending balance	(9)	62
Amount expected to be incurred in future periods	11
Contract Termination Costs [Member] | Restructuring Plan, July 2018 [Member]
Restructuring Cost and Reserve [Line Items]
Amounts accrued, Beginning balance	94
Charges for the year		1,108
Revised estimates during the year/ period	12	187
Total restructuring costs expected to be incurred		1,295
Amounts paid during the year/ period	(78)	(1,201)
Amounts accrued, Ending balance	$ 28	94
San Bruno Office Closing Costs [Member] | Restructuring Plan, July 2018 [Member]
Restructuring Cost and Reserve [Line Items]
Charges for the year		465
Revised estimates during the year/ period		5
Total restructuring costs expected to be incurred		470
Amounts paid during the year/ period		$ (470)