Convertible Debt and Notes Payable - Convertible Debt (Details) - USD ($)	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. 14, 2017
Debt and Capital Lease Obligations [Line Items]
Convertible debt	$ 0		$ 0		$ 80,808,000	[1]
Amortization of debt discount (premium)			6,695,000	$ 3,393,000
Gain/(loss) on debt extinguishment	0	$ 0	(4,538,000)	0
Convertible Debt
Debt and Capital Lease Obligations [Line Items]
Unamortized discount					6,700,000
2022 convertible note | Convertible Debt
Debt and Capital Lease Obligations [Line Items]
Debt Instrument, Face Amount							$ 135,000,000
Debt Instrument, Convertible, Conversion Price							$ 17.375
Interest on Convertible Debt, Net of Tax		1,000,000	1,300,000	3,000,000
Unamortized discount	5,100,000		5,100,000				$ 23,400,000
Amortization of debt discount (premium)		$ 1,200,000	1,500,000	$ 3,500,000
Debt Instrument, Interest Rate, Effective Percentage							7.13%
Debt Issuance Costs, Gross							$ 2,200,000
Extinguishment of Debt, Amount					70,500,000
Gain/(loss) on debt extinguishment					2,700,000
Debt Instrument, Repurchased Face Amount	88,100,000		$ 88,100,000		46,900,000
Embedded Derivative, No Longer Bifurcated, Amount Reclassified to Stockholders' Equity					29,600,000
Write off of Deferred Debt Issuance Cost					400,000
Write off of pro rate unamortized discount while extinguishing debt					$ 3,600,000
Debt Instrument, Convertible, Threshold Percentage of Stock Price Trigger			33.35%
Debt Issuance Costs, Net	$ 500,000		$ 500,000
2022 convertible note | Convertible Debt | Common Stock Class A
Debt and Capital Lease Obligations [Line Items]
Debt Conversion, Converted Instrument, Shares Issued			2,938,841

[1] The carrying amount of the convertible debt includes an unamortized discount of $6.7 million as of December 31, 2020.