Income Taxes - Reconciliation (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Sep. 30, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Taxes
Effective tax rates on continuing operations		21.60%	24.70%	24.10%
Federal statutory income tax rate (as a percent)		21.00%	21.00%	21.00%
Income taxes at the federal statutory rate		$ 40,223	$ 31,866	$ 31,222
Increases (decreases) resulting from-
Net state income taxes		8,406	6,644	7,470
Valuation allowances		(254)	(279)	(2,852)
Net unrecognized tax benefits		18,557	7,338	(15)
Nondeductible expenses		2,470	2,180	1,926
R&D tax credits	$ (4,600)	(26,133)	(4,569)	(2,726)
179D deduction		(1,062)	(5,126)
Net operating loss carryforwards				2,225
Stock-based compensation deductions		(426)	(714)	(1,293)
Other		(380)	78	(184)
Provision for income taxes		$ 41,401	37,418	$ 35,773
179D deduction			$ (2,200)

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Income tax reconciliation section 179D deduction.

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Income tax reconciliation section 179D deduction, net of tax contingencies.

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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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Amount of difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying domestic federal statutory income tax rate to pretax income (loss) from continuing operation, attributable to expense for award under share-based payment arrangement. Excludes expense determined to be nondeductible upon grant or after for award under share-based payment arrangement.

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

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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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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 tax exempt income, equity in earnings (loss) of an unconsolidated subsidiary, minority noncontrolling interest income (loss), tax holiday, disposition of a business, disposition of an asset, repatriation of foreign earnings, repatriation of foreign earnings jobs creation act of 2004, increase (decrease) in enacted tax rate, prior year income taxes, increase (decrease) in deferred tax asset valuation allowance, and other adjustments.

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