Restructuring - Summary of Total Amount Expected to Be Incurred and Liability (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($)
Restructuring Plan, July 2018 [Member]
Restructuring Cost and Reserve [Line Items]
Charges for the year	$ 9,155
Revised estimates during the year	235
Total restructuring costs expected to be incurred	9,390
Amounts paid during the year	(5,756)
Amounts accrued, Ending balance	3,634
Amounts accrued at December 31, 2018	3,634
Clinical Trial Closing Costs [Member] | Restructuring Plan, July 2018 [Member]
Restructuring Cost and Reserve [Line Items]
Charges for the year	5,703
Revised estimates during the year	41
Total restructuring costs expected to be incurred	5,744
Amounts paid during the year	(2,204)
Amounts accrued, Ending balance	3,540
Amounts accrued at December 31, 2018	3,540
One-time Employee Termination Benefits [Member] | Restructuring Plan, November 2018 [Member]
Restructuring Cost and Reserve [Line Items]
Total restructuring costs expected to be incurred	984
Amounts paid during the year	(922)
Amounts accrued, Ending balance	62
Amounts accrued at December 31, 2018	62
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	2
Total restructuring costs expected to be incurred	1,881
Amounts paid during the year	(1,881)
Contract Termination Costs [Member] | Restructuring Plan, July 2018 [Member]
Restructuring Cost and Reserve [Line Items]
Charges for the year	1,108
Revised estimates during the year	187
Total restructuring costs expected to be incurred	1,295
Amounts paid during the year	(1,201)
Amounts accrued, Ending balance	94
Amounts accrued at December 31, 2018	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	5
Total restructuring costs expected to be incurred	470
Amounts paid during the year	$ (470)