Commitments and Contingencies - Additional Information (Details)			9 Months Ended
	Jan. 25, 2021 USD ($) ft²	Jul. 01, 2019 USD ($) ft²	Sep. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)
Commitments And Contingencies [Line Items]
Operating lease right-of-use assets			$ 63,841,000	$ 60,835,000
Restricted cash			$ 16,130,000	$ 10,627,000
Car Offer \| Addison, Texas at 15601 Dallas Parkway
Commitments And Contingencies [Line Items]
Operating lease, office space \| ft²	61,826
Sublease term description			non-cancellable lease term through 2030
Sublease commencement date	Mar. 01, 2021
Sublease rent commencement date	Jan. 01, 2022
Sublease monthly base rent payable	$ 152,000
Increase in sublease monthly base rent, maximum amount payable	$ 185,000
Car Offer \| Addison, Texas at 15601 Dallas Parkway \| Furniture and Fixtures
Commitments And Contingencies [Line Items]
Operating lease right-of-use assets percentage	5.00%
Operating lease right-of-use assets	$ 664,000
Car Offer \| Addison, Texas at 15601 Dallas Parkway \| Office Space
Commitments And Contingencies [Line Items]
Operating lease right-of-use assets percentage	95.00%
Operating lease right-of-use assets	$ 12,336,000
Car Offer \| Plano, Texas at 2701 East Plano Parkway
Commitments And Contingencies [Line Items]
Operating lease, office space \| ft²		26,971
Sublease term description			non-cancellable lease term through 2023
Sublease commencement date		Jul. 01, 2019
Sublease rent commencement date		Jul. 01, 2019
Sublease monthly base rent payable		$ 21,000
Increase in sublease monthly base rent, maximum amount payable		$ 22,000