Restructuring - Summary of Total Amount Expected to Be Incurred and Liability (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Restructuring Plan, July 2018 [Member]
Restructuring Cost and Reserve [Line Items]
Charges			$ 9,155
Revised estimates		$ (1,945)	235
Total restructuring costs expected to be incurred		7,400	9,390
Amounts paid		(1,689)	(5,756)
Amounts accrued, Ending balance			3,634
Clinical Trial Closing Costs [Member] | Restructuring Plan, July 2018 [Member]
Restructuring Cost and Reserve [Line Items]
Charges			5,703
Revised estimates	$ 2,000	(1,957)	41
Total restructuring costs expected to be incurred			5,744
Amounts paid		(1,583)	(2,204)
Amounts accrued, Ending balance			3,540
One-time Employee Termination Benefits [Member] | Restructuring Plan, July 2018 [Member]
Restructuring Cost and Reserve [Line Items]
Charges			1,879
Revised estimates			2
Total restructuring costs expected to be incurred			1,881
Amounts paid			(1,881)
Amounts accrued, Ending balance
One-time Employee Termination Benefits [Member] | Restructuring Plan, November 2018 [Member]
Restructuring Cost and Reserve [Line Items]
Charges		655
Total restructuring costs expected to be incurred		1,645	984
Amounts paid		(702)	(922)
Amounts accrued, Ending balance		15	62
Cumulative restructuring costs incurred		(1,639)
Amount expected to be incurred in future periods		6
Contract Termination Costs [Member] | Restructuring Plan, July 2018 [Member]
Restructuring Cost and Reserve [Line Items]
Charges			1,108
Revised estimates		12	187
Total restructuring costs expected to be incurred			1,295
Amounts paid		(106)	(1,201)
Amounts accrued, Ending balance			94
San Bruno Office Closing Costs [Member] | Restructuring Plan, July 2018 [Member]
Restructuring Cost and Reserve [Line Items]
Charges			465
Revised estimates			5
Total restructuring costs expected to be incurred			470
Amounts paid			$ (470)
Amounts accrued, Ending balance