Income Taxes - Reconciliation of U.S. Federal Statutory Income Tax Rate to Effective Income Tax Rate (Details)			12 Months Ended
Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Federal statutory income tax rate	21.00%	35.00%	21.00%	21.00%	34.00%
State taxes, net of federal benefit			7.00%	6.10%	1.60%
Research and orphan drug credit			5.40%	9.60%	7.70%
Research and orphan drug credit addback			0.00%	0.00%	(6.60%)
Impact of change in tax status			0.00%	0.00%	(8.70%)
Effect of federal tax law change			0.00%	0.00%	(6.10%)
Permanent adjustments and other			3.80%	0.70%	(0.30%)
Increase in deferred tax asset valuation allowance			(37.20%)	(37.40%)	(21.60%)
Effective income tax rate			0.00%	0.00%	0.00%

Effective income tax rate reconciliation, impact of change in tax status.

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Percentage of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic statutory income tax rates to pretax income (loss) from continuing operations attributable to research and orphan drug credit.

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Percentage of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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Percentage of domestic federal statutory tax rate applicable to pretax income (loss).

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Percentage 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 deductions. Including, but not limited to, extraterritorial income exclusion deduction, qualified production activity deduction, dividend deduction, deduction for dividend paid to employee stock ownership plan, Medicare prescription drug benefit subsidy deduction, and other deductions.

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