Income Taxes - Reconciliation of U.S. Statutory Income Tax Rate to Effective Tax Rate (Details)	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of the U.S. statutory income tax rate to the Company's effective tax rate
Federal income tax at statutory rate (as a percent)	21.00%	34.00%	34.00%
State income tax benefit, net of federal benefit (as a percent)	6.00%	6.00%	6.00%
Research and development tax credits (as a percent)	3.00%	3.00%	2.00%
Effect of tax rate changes (as a percent)	0.00%	(94.00%)	0.00%
Other (as a percent)	(4.00%)	(1.00%)	1.00%
Decrease (increase) to valuation allowance (as a percent)	(24.00%)	52.00%	(33.00%)
Effective income tax rate (as a percent)	2.00%	0.00%	10.00%

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 changes in the valuation allowance for deferred tax assets.

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Percentage of the difference, between reported income tax expense (benefit) and the expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations, that is attributable to tax exempt income, equity in earnings (loss) of an unconsolidated subsidiary, minority interest income (expense), tax holiday, disposition of a business, disposition of an asset, repatriation of foreign earnings, repatriation of foreign earnings jobs creation act of 2004, change in enacted tax rate, prior year income taxes, change in deferred tax asset valuation allowance, and other adjustments.

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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 applicable to state and local income tax expense (benefit), net of federal tax expense (benefit).

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