Summary of Significant Accounting Policies - Additional Information (Detail) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2022 USD ($) credit_facility loan $ / shares	Sep. 30, 2021 USD ($) $ / shares	Sep. 30, 2022 USD ($) credit_facility loan $ / shares	Sep. 30, 2021 USD ($) $ / shares	Jun. 30, 2022 USD ($)	Mar. 31, 2022 USD ($)	Jan. 01, 2022 USD ($)	Dec. 31, 2021 USD ($)	Jun. 30, 2021 USD ($)	Mar. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Apr. 30, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Joint venture capital contribution percentage												85.00%
Borrower escrows	$ 498,800		$ 498,800					$ 531,200
CECL reserve	144,431	$ 130,388	144,431	$ 130,388	$ 133,024	$ 122,221		124,679	$ 128,945	$ 172,100	$ 173,549
Principal balance	24,423,133		24,423,133					22,003,017
Decrease to additional paid-in capital	(5,467,372)		(5,467,372)					(5,373,029)
Decrease to accumulated deficit	$ (814,505)		$ (814,505)					$ (794,832)
Decrease to diluted earnings per share (in dollars per share) | $ / shares	$ (0.59)	$ (0.56)	$ (1.71)	$ (2.00)
Credit Spread Option | Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans | loan	49		49
Number of credit facilities | credit_facility	9		9
Average compounded SOFR (percentage)	3.04%		3.04%
Credit Spread Option | United State Dollar LIBOR Rate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average compounded SOFR (percentage)	3.14%		3.14%
Level 3 | Measurement Input, Cap Rate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Measurement input (percentage)	0.0450		0.0450
Level 3 | Measurement Input, Discount Rate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Measurement input (percentage)	0.0750		0.0750
Level 3 | Fair Value, Nonrecurring
Accounts, Notes, Loans and Financing Receivable [Line Items]
CECL reserve	$ 54,900		$ 54,900
Principal balance	$ 286,300		$ 286,300
Accounting Standards Update 2020-06 | Cumulative Effect, Period of Adoption, Adjustment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Decrease to additional paid-in capital							$ 2,400
Decrease to accumulated deficit							2,000
Increase to convertible notes, net							$ 476
Decrease to diluted earnings per share (in dollars per share) | $ / shares			$ 0.03
Walker and Dunlop
Accounts, Notes, Loans and Financing Receivable [Line Items]
Joint venture capital contribution percentage												15.00%