Income Taxes (Details 3) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Unrecognized tax benefits	$ 0
Penalties and interest expense	0	$ 0	$ 0
Reconciliation of income taxes computed using the U.S. federal statutory rate to that reflected in operations
Income tax benefit using U.S. federal statutory rate	(62,141,000)	(34,391,000)	(21,243,000)
State income taxes, net of federal benefit	(5,236,000)	(4,434,000)	(2,494,000)
Stock-based compensation	1,585,000	752,000	149,000
Research and development tax credits	(2,794,000)	(1,469,000)	(499,000)
Change in the valuation allowance	48,096,000	39,291,000	23,186,000
Permanent items	53,000	26,000	910,000
Other	1,371,000	225,000	(9,000)
Expiring NOLs and credits - 382 Limitation	19,066,000	0	0
Income tax expense (benefit)	0	0	$ 0
Non-current assets:
NOL carryforwards	193,436,000	154,239,000
Capitalized research and development	1,970,000	263,000
Research and development credits	4,525,000	6,313,000
Depreciation and amortization	(173,000)	(119,000)
Accrued expenses	3,109,000	1,073,000
Stock-based compensation	14,903,000	7,753,000
Other	55,000	29,000
Gross non-current deferred tax assets	217,825,000	169,551,000
Valuation allowance	(217,825,000)	(169,551,000)
Net non-current deferred tax assets	$ 0	$ 0