Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jan. 01, 2017	Dec. 31, 2014
Operating Loss Carryforwards [Line Items]
Cumulative change in ownership		More than 50% occurs within a three-year period.
Deferred tax assets		$ 83,610,000	$ 114,764,000
Percentage of cumulative change in ownership		50.00%
Valuation allowance		$ 83,610,000	114,764,000
Tax cuts and jobs act of 2017 change in deferred tax assets valuation allowance		$ 2,020,000
U.S. federal corporate tax rate		35.00%
Tax cuts and jobs act of 2017, accounting complete [true false]		false
Tax cuts and jobs act of 2017, incomplete accounting, change in tax rate, provisional income tax expense (benefit)		$ 34,439,000	0	$ 0
Gross unrecognized tax benefits		17,836,000	17,561,000	4,340,000		$ 582,000
Accrual for interest or penalties		0	0
Interest or penalties expense		$ 0	0	$ 0
Tax year open to examination		1998
Scenario, Forecast [Member]
Operating Loss Carryforwards [Line Items]
U.S. federal corporate tax rate	21.00%
ASU 2016-09 [Member]
Operating Loss Carryforwards [Line Items]
Adoption of new accounting pronouncement of unrecognized excess tax benefits					$ 600,000
Domestic Tax Authority [Member]
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards		$ 101,200,000
Research and development credits and other tax credits		12,200,000
Net operating loss carryforwards		318,900,000	316,000,000
Federal research and development credit and orphan drug credit carryforwards		$ 26,300,000	26,300,000
Domestic Tax Authority [Member] | Earliest Tax Year [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards, expiration year		2018
Federal research and development credit and orphan drug credit carryforwards, expiration year		2018
Domestic Tax Authority [Member] | Latest Tax Year [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards, expiration year		2037
Federal research and development credit and orphan drug credit carryforwards, expiration year		2033
State and Local Jurisdiction [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards		$ 254,600,000	269,600,000
Federal research and development credit and orphan drug credit carryforwards		$ 8,800,000	$ 8,800,000
State and Local Jurisdiction [Member] | Earliest Tax Year [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards, expiration year		2028
State and Local Jurisdiction [Member] | Latest Tax Year [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards, expiration year		2036