Summary of Significant Accounting Policies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2021 USD ($) credit_facility loan	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Jan. 01, 2022 USD ($)	Apr. 30, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Joint venture capital contribution percentage					85.00%
Restricted cash	$ 0	$ 0
Borrower escrows	531,200,000	384,600,000
CECL reserve recorded	(39,864,000)	167,653,000	$ 0
Amount charged off	14,427,000
CECL reserve	124,679,000	173,549,000	0
Principal balance	22,003,017,000	16,572,715,000	16,164,801,000
Decrease to additional paid-in capital	(5,373,029,000)	(4,702,713,000)
Decrease to accumulated deficit	$ (794,832,000)	(829,284,000)
Credit Spread Option | Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans | loan	16
Number of credit facilities | credit_facility	1
Credit spread adjustment rate	0.0011
Average compounded SOFR	0.05%
Credit Spread Option | United State Dollar LIBOR Rate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average compounded SOFR	0.10%
Credit Spread Option | Sterling Overnight Interbank Average Rate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans | loan	9
Number of credit facilities | credit_facility	5
Daily compounded SONIA	0.19%
Credit Spread Option | Great Britain Pound LIBOR Rate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Daily compounded SONIA	0.26%
Impaired loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount charged off	$ 14,427,000
CECL reserve	54,874,000	69,661,000	0
Principal balance	$ 284,809,000	$ 337,197,000
Level 3 | Measurement Input, Cap Rate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Measurement input	0.0480
Level 3 | Measurement Input, Discount Rate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Measurement input	0.0830
Level 3 | Fair Value, Nonrecurring
Accounts, Notes, Loans and Financing Receivable [Line Items]
CECL reserve recorded	$ 69,700,000
Amount charged off	14,400,000
CECL reserve	54,900,000
Principal balance	$ 286,300,000
Cumulative Effect, Period of Adoption, Adjustment
Accounts, Notes, Loans and Financing Receivable [Line Items]
CECL reserve			$ 17,650,000
Accounting Standards Update 2020-06 | Cumulative Effect, Period of Adoption, Adjustment | Forecast | Subsequent Event
Accounts, Notes, Loans and Financing Receivable [Line Items]
Decrease to additional paid-in capital				$ 2,400,000
Decrease to accumulated deficit				2,000,000.0
Convertible Notes Payable				$ 476,000
Walker and Dunlop
Accounts, Notes, Loans and Financing Receivable [Line Items]
Joint venture capital contribution percentage					15.00%