Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Tax [Line Items]
Total provision (benefit) for income taxes	$ 0	$ (365,000)
Increase (Decrease) in valuation allowance	114,200,000	67,000,000
Unrecognized income tax benefits that, if recognized, would affect effective tax rate	0	0
Unrecognized income tax benefits	13,866,000	$ 3,228,000	$ 2,906,000	$ 2,048,000
Federal
Income Tax [Line Items]
Net operating loss carryforwards	694,800,000
Excess stock option benefits	$ 41,900,000
Federal | Minimum
Income Tax [Line Items]
Net operating loss carryforwards, expiration year	2024
Federal | Maximum
Income Tax [Line Items]
Net operating loss carryforwards, expiration year	2036
Federal | Research Tax Credit Carryforward
Income Tax [Line Items]
Tax credits carryforwards	$ 16,500,000
Federal | Orphan Drug Credit
Income Tax [Line Items]
Tax credits carryforwards	71,600,000
California
Income Tax [Line Items]
Net operating loss carryforwards	$ 223,500,000
California | Minimum
Income Tax [Line Items]
Net operating loss carryforwards, expiration year	2017
California | Maximum
Income Tax [Line Items]
Net operating loss carryforwards, expiration year	2032
California | Research Tax Credit Carryforward
Income Tax [Line Items]
Tax credits carryforwards	$ 6,400,000