Summary of Significant Accounting Policies - Additional Information (Detail) $ in Thousands	Sep. 30, 2023 USD ($) loan	Jun. 30, 2023 USD ($)	Mar. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Sep. 30, 2022 USD ($)	Jun. 30, 2022 USD ($)	Mar. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Joint venture capital contribution percentage									85.00%
Restricted cash	$ 0			$ 0
Borrower escrows	634,700			459,600
CECL reserve	463,423	$ 363,875	$ 336,591	326,137	$ 144,431	$ 133,024	$ 122,221	$ 124,679
Principal balance	23,715,901			25,017,880
Impaired loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
CECL reserve	322,551	$ 214,401	$ 197,258	189,778	$ 54,874	$ 54,874	$ 54,874	$ 54,874
Principal balance	1,391,522			$ 929,111
Ten Loans | Impaired loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
CECL reserve	$ 322,600
Level 3 | Measurement Input, Cap Rate | Minimum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Measurement input (percentage)	0.0525
Level 3 | Measurement Input, Cap Rate | Maximum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Measurement input (percentage)	0.0780
Level 3 | Measurement Input, Discount Rate | Minimum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Measurement input (percentage)	0.0728
Level 3 | Measurement Input, Discount Rate | Maximum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Measurement input (percentage)	0.1000
Level 3 | Fair Value, Nonrecurring
Accounts, Notes, Loans and Financing Receivable [Line Items]
CECL reserve	$ 322,600
Number of loans accounted for under cost-recovery method, floating rate | loan	10
Principal balance	$ 1,400,000
Walker and Dunlop
Accounts, Notes, Loans and Financing Receivable [Line Items]
Joint venture capital contribution percentage									15.00%