Restructuring Costs - Summary of Restructuring Activities (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2017
Restructuring Cost and Reserve [Line Items]
Accrual, Beginning balance		$ 763
Accrual, Ending balance	$ 110	110
PNT2258 [Member]
Restructuring Cost and Reserve [Line Items]
Accrual, Beginning balance	206	763
Cash payments	(58)	(541)
Accrual, Ending balance	110	110
PNT2258 [Member] | Contract Termination [Member]
Restructuring Cost and Reserve [Line Items]
Accrual, Beginning balance	204	664
Cash payments	(56)	(442)
Accrual, Ending balance	110	110
PNT2258 [Member] | Employee Termination [Member]
Restructuring Cost and Reserve [Line Items]
Accrual, Beginning balance	2	99
Cash payments	(2)	(99)
Research and development [Member] | PNT2258 [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring cost	(38)	(112)
Research and development [Member] | PNT2258 [Member] | Contract Termination [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring cost	$ (38)	$ (112)