Income Taxes - Net deferred tax assets and liabilities (Details) - USD ($)	Dec. 31, 2016	Dec. 31, 2015
Deferred tax assets
Net operating loss carryforwards	$ 78,021,000	$ 73,678,000
Research and development credit carryforwards	14,143,000	12,398,000
Orphan drug credit	7,235,000	6,815,000
Stock options	7,105,000	5,568,000
Other	562,000	235,000
Deferred tax assets	107,066,000	98,694,000
Deferred tax liabilities
Deferred revenue		(8,049,000)
Property and equipment	(337,000)	(361,000)
Deferred tax liabilities	(337,000)	(8,410,000)
Net deferred tax asset before valuation allowance	106,729,000	90,284,000
Less valuation allowance	$ (106,729,000)	$ (90,284,000)