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Income Taxes - Reconciliation (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of federal statutory income tax to effective income tax
Federal statutory income tax rate	$ (16,917,000)	$ 7,718,000	$ (6,157,000)
State and local income taxes	(2,436,000)	1,451,000
Research and development credit	(5,554,000)	(2,544,000)	708,000
Stock based compensation	2,709,000	733,000	651,000
Effect of the 2017 Tax Cuts and Jobs Act	23,859,000
Other	262,000	8,000	14,000
Net change in valuation allowance	(2,386,000)	(6,366,000)	4,784,000
Income tax provision	$ (463,000)	$ 1,000,000
State
Reconciliation of federal statutory income tax to effective income tax
Current Income Tax Expense (Benefit)			0
Federal
Reconciliation of federal statutory income tax to effective income tax
Current Income Tax Expense (Benefit)			$ 0

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Amount of current income tax expense (benefit) pertaining to taxable income (loss) from continuing operations.

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Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to increase (decrease) in the valuation allowance for deferred tax assets.

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The amount of income tax expense or benefit for the period computed by applying the domestic federal statutory tax rates to pretax income from continuing operations.

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