Restructuring Costs - Summary of Restructuring Activities (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Restructuring Cost and Reserve [Line Items]
Accrual, Beginning balance	$ 763
Non-cash charges		$ (811)
Accrual, Ending balance	137	763
PNT2258 [Member]
Restructuring Cost and Reserve [Line Items]
Accrual, Beginning balance	763
Adjustment to research and development expense	(69)
Non-cash charges		(811)
Cash payments	(557)	(1,142)
Accrual, Ending balance	137	763
PNT2258 [Member] | Contract Termination [Member]
Restructuring Cost and Reserve [Line Items]
Accrual, Beginning balance	664
Adjustment to research and development expense	(69)
Non-cash charges		(681)
Cash payments	(458)	(909)
Accrual, Ending balance	137	664
PNT2258 [Member] | Employee Termination [Member]
Restructuring Cost and Reserve [Line Items]
Accrual, Beginning balance	99
Cash payments	$ (99)	(233)
Accrual, Ending balance		99
PNT2258 [Member] | Asset Impairment [Member]
Restructuring Cost and Reserve [Line Items]
Non-cash charges		(130)
Research and development [Member] | PNT2258 [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring cost		2,249
Research and development [Member] | PNT2258 [Member] | Contract Termination [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring cost		2,249
General and administrative [Member] | PNT2258 [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring cost		467
General and administrative [Member] | PNT2258 [Member] | Contract Termination [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring cost		5
General and administrative [Member] | PNT2258 [Member] | Employee Termination [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring cost		332
General and administrative [Member] | PNT2258 [Member] | Asset Impairment [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring cost		$ 130