Income Taxes - Income Tax Rate Reconciliation (Details)	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Federal statutory rate	35.00%	35.00%	35.00%
State income tax expense, net of federal benefits	0.10%	4.90%	4.50%
Nondeductible meals and entertainment	0.60%	1.60%	1.20%
Research and development tax credits	(16.10%)	(10.80%)	(8.90%)
Tax windfall benefits	(36.50%)	(0.00%)	(0.00%)
Change in tax rate due to tax reform	27.20%	0.00%	0.00%
Other	(1.00%)	(1.30%)	0.80%
Effective rate	9.30%	29.40%	32.60%

Effective Income Tax Rate Reconciliation, Tax Windfall Benefits

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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 changes in the income tax rates.

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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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