Summary of Significant Accounting Policies (Detail)	3 Months Ended
	Mar. 31, 2024 USD ($) real_estate_owned_asset loan	Mar. 19, 2024 USD ($)	Dec. 31, 2023 USD ($)	Mar. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Joint venture capital contribution percentage						85.00%
Restricted cash	$ 0		$ 0
Number of REO assets classified as held for sale investment | real_estate_owned_asset	1
Borrower escrows	$ 582,300,000		640,600,000
CECL reserve	751,370,000		576,936,000	$ 336,591,000	$ 326,137,000
Principal balance	23,189,312,000		23,787,012,000
Real estate owned, net	$ 60,203,000		0
Building
Accounts, Notes, Loans and Financing Receivable [Line Items]
Estimated useful lives (in years)	40 years
Leasehold Improvements
Accounts, Notes, Loans and Financing Receivable [Line Items]
Estimated useful lives (in years)	10 years
Impaired loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
CECL reserve	$ 602,599,000		417,670,000	$ 197,258,000	$ 189,778,000
Principal balance	$ 2,390,680,000		$ 1,877,279,000
Level 3 | Measurement Input, Cap Rate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Real estate acquired through foreclosure, measurement input		0.0700
Level 3 | Measurement Input, Cap Rate | Minimum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Measurement input (percentage)	0.0600
Level 3 | Measurement Input, Cap Rate | Maximum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Measurement input (percentage)	0.0855
Level 3 | Measurement Input, Discount Rate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Real estate acquired through foreclosure, measurement input		0.0950
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)	0.1100
Level 3 | Fair Value, Nonrecurring
Accounts, Notes, Loans and Financing Receivable [Line Items]
CECL reserve	$ 602,600,000
Number of loans accounted for under cost-recovery method, floating rate | loan	17
Principal balance	$ 2,400,000,000
Real estate owned, net		$ 60,200,000
Walker and Dunlop
Accounts, Notes, Loans and Financing Receivable [Line Items]
Joint venture capital contribution percentage						15.00%