Summary of Significant Accounting Policies (Details)	9 Months Ended			12 Months Ended
	Sep. 30, 2024 USD ($) real_estate_owned_asset	Sep. 30, 2024 USD ($) loan real_estate_owned_asset	Sep. 30, 2024 USD ($) real_estate_owned_asset security_loan	Dec. 31, 2023 USD ($) loan	Dec. 31, 2023 USD ($) security_loan	Sep. 16, 2024 USD ($)	Jul. 02, 2024 USD ($)	Jun. 30, 2024 USD ($)	Mar. 31, 2024 USD ($)	Mar. 19, 2024 USD ($)	Sep. 30, 2023 USD ($)	Jun. 30, 2023 USD ($)	Mar. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Restricted cash	$ 0	$ 0	$ 0	$ 0	$ 0
Borrower escrows	$ 611,900,000	$ 611,900,000	$ 611,900,000	640,600,000	640,600,000
Number of REO assets classified as held for sale investment | real_estate_owned_asset	3	3	3
CECL reserve	$ 1,011,059,000	$ 1,011,059,000	$ 1,011,059,000	$ 576,936,000	$ 576,936,000			$ 893,938,000	$ 751,370,000		$ 463,423,000	$ 363,875,000	$ 336,591,000	$ 326,137,000
Number of loans		149	149	178	178
Principal balance	21,602,517,000	$ 21,602,517,000	$ 21,602,517,000	$ 23,787,012,000	$ 23,787,012,000
Real estate owned, net	$ 138,725,000	$ 138,725,000	$ 138,725,000	$ 0	$ 0
Multifamily Joint Venture
Accounts, Notes, Loans and Financing Receivable [Line Items]
Ownership percentage															85.00%
Multifamily Joint Venture | Walker & Dunlop
Accounts, Notes, Loans and Financing Receivable [Line Items]
Minority participation in senior term facility															15.00%
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
Level 3 | Measurement Input, Cap Rate | Minimum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Measurement input	0.0600	0.0600	0.0600
Level 3 | Measurement Input, Cap Rate | Maximum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Measurement input	0.0855	0.0855	0.0855
Level 3 | Measurement Input, Discount Rate | Minimum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Measurement input	0.0728	0.0728	0.0728
Level 3 | Measurement Input, Discount Rate | Maximum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Measurement input	0.1100	0.1100	0.1100
Level 3 | Fair Value, Nonrecurring
Accounts, Notes, Loans and Financing Receivable [Line Items]
CECL reserve	$ 883,600,000	$ 883,600,000	$ 883,600,000
Number of loans | loan		20
Principal balance	$ 3,200,000,000	$ 3,200,000,000	$ 3,200,000,000
Level 3 | Fair Value, Nonrecurring | Mountain View, California Office Property
Accounts, Notes, Loans and Financing Receivable [Line Items]
Real estate owned, net										$ 60,200,000
Level 3 | Fair Value, Nonrecurring | San Antonio, Texas Multifamily Property
Accounts, Notes, Loans and Financing Receivable [Line Items]
Real estate owned, net							$ 33,600,000
Level 3 | Fair Value, Nonrecurring | Burlington, Massachusetts Office Property
Accounts, Notes, Loans and Financing Receivable [Line Items]
Real estate owned, net						$ 64,600,000
Level 3 | Fair Value, Nonrecurring | Measurement Input, Cap Rate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Real estate acquired through foreclosure, measurement input						0.0800	0.0600			0.0700
Level 3 | Fair Value, Nonrecurring | Measurement Input, Discount Rate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Real estate acquired through foreclosure, measurement input						0.1000	0.0750			0.0950