Income Taxes - Schedule of Loss Before Income Taxes (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended	12 Months Ended
Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Pre-tax book loss	$ (30,500)	$ (12,317)	$ (53,349)	$ (39,187)	$ (48,990)	$ (53,295)
Tax at federal statutory rate					(10,288)	(11,192)
Stock-based compensation					3	1,060
State taxes					879	766
Foreign rate differential					(1,223)	(871)
Unrecognized tax benefit					549
Other adjustments					1,453	(1,707)
Valuation allowance					9,563	12,663
Total income tax provision	$ (969)	$ 390	$ (97)	$ 964	$ 936	$ 719

Income tax reconciliation unrecognized tax benefit.

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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 income (loss) from continuing operations, including income (loss) from equity method investments, before deduction of income tax expense (benefit), and income (loss) attributable to noncontrolling interest.

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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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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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