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Loans and Investments - Narrative (Details) $ in Thousands	1 Months Ended		3 Months Ended				12 Months Ended
	Sep. 30, 2023 USD ($)	Apr. 30, 2023 USD ($) loan	Mar. 31, 2024 USD ($) loan borrower	Dec. 31, 2023 USD ($) loan	Jun. 30, 2023 USD ($)	Mar. 31, 2023 USD ($) loan	Dec. 31, 2023 USD ($) loan borrower	Dec. 31, 2022 USD ($)
Loans and Investments
Number of loans | loan			781				747
Provision for credit losses			$ 17,778			$ 20,518
Accrued interest receivable related to loans			$ 131,000	$ 124,200			$ 124,200
Number of loans for which no provision for loan loss made | loan			0	0			0
Interest income recognized on nonaccrual loans			$ 8,700			600
Loans and investments, allowance for credit losses			$ 211,942	$ 195,664		$ 153,077	$ 195,664	$ 132,559
Number of impaired loans | loan			22	19			19
Future funding commitments, modified loans, borrowers experiencing financial difficulty			$ 28,000
Multifamily bridge loan			12,249,862	$ 12,615,006			$ 12,615,006
Interest reserve			$ 167,600				156,100
Loan modification, refinancing, and extensions | loan			0			0
Number of loans sold | loan		4
Bridge loans, total UPB			$ 507,659				$ 427,929
Number of loans covered under interest reserve | loan			572	537			537
Aggregate UPB covered under interest reserve			$ 8,470,000	$ 8,440,000			$ 8,440,000
Bridge loans
Loans and Investments
Number of loans | loan			698				679
Variable rate, spread					3.40%
Financing receivable, accrual rate	0.0300
Multifamily bridge loan			$ 11,866,289	12,273,244	$ 70,500		$ 12,273,244
Financing receivable, fixed interest rate	0.0300
Financing receivable, total fixed rate	0.0600
Financing receivable, borrower required funding	$ 10,500
Interest reserve	2,500
Financing receivable, borrower requirement, capital improvements	$ 8,000
Bridge loans, total UPB		$ 217,400
Six loans collateralized by a land development project
Loans and Investments
Number of loans with unpaid principal balance | loan			6
Unpaid principal balance on loans			$ 121,400
Five loans collateralized by a land development project
Loans and Investments
Number of loans with unpaid principal balance | loan			5
Unpaid principal balance on loans			$ 112,100
Weighted average accrual rate of interest (as a percent)			7.91%
Loans collateralized by a land development project
Loans and Investments
Loans and investments, allowance for credit losses			$ 71,400	71,400			$ 71,400
Non-performing loans
Loans and Investments
Number of loans | loan			21				16
Carrying value of loans			$ 406,100	235,600			$ 235,600
Loan loss reserves			32,900	27,100			27,100
Bridge loans, total UPB			464,827				274,152
Non-performing loans | Greater Than 90 Days Past Due
Loans and Investments
Interest receivable			0	0			0
Multifamily bridge loan			464,827	274,152			274,152
Land
Loans and Investments
Provision for credit losses			62			$ 18
Outstanding unfunded commitments			1,590,000	1,310,000			1,310,000
Loans and investments, allowance for credit losses			78,120	78,058		78,086	78,058	78,068
Bridge loans, total UPB			134,215				134,215
Multifamily
Loans and Investments
Provision for credit losses			16,652			20,387
Loans and investments, allowance for credit losses			125,999	$ 110,847		58,348	110,847	37,961
Bridge loans, total UPB			352,223				272,493
Multifamily | Bridge loans
Loans and Investments
Loans previously classified as non-performing			$ 712,900
Number of financing receivables modified during the period | loan			16
Financing receivable, amount modified in period			$ 692,800
Weighted average pay rate			0.0695
Financing receivable, accrual rate			0.0186
Multifamily | Bridge loans | Minimum | Secured Overnight Financing Rate (SOFR)
Loans and Investments
Variable rate, spread			3.25%
Multifamily | Bridge loans | Maximum | Secured Overnight Financing Rate (SOFR)
Loans and Investments
Variable rate, spread			4.25%
Multifamily | Bridge loans | Temporary Interest Rate Relief, Twenty Three Loans
Loans and Investments
Number of financing receivables modified during the period | loan			23
Financing receivable, amount modified in period			$ 1,070,000
Multifamily | Bridge loans | Temporary Interest Rate Relief
Loans and Investments
Number of impaired loans | loan			2
Loan loss reserve, specifically impaired loans			$ 7,000
Total UPB, specifically impaired loans			49,600
Multifamily | Bridge loans | Temporary Interest Rate Relief And Extended Maturity
Loans and Investments
Financing receivable, amount modified in period			$ 671,000
Number of financing receivables with extended maturities | loan			15
Multifamily | Bridge loans | Temporary Interest Rate Relief And Extended Maturity | Minimum
Loans and Investments
Financing receivable modified, weighted average term increase from modification				12 months
Multifamily | Bridge loans | Temporary Interest Rate Relief And Extended Maturity | Maximum
Loans and Investments
Financing receivable modified, weighted average term increase from modification				30 months
Multifamily | Bridge loans | Extended Maturity
Loans and Investments
Number of financing receivables modified during the period | loan			11
Financing receivable, amount modified in period			$ 456,500
Multifamily | Bridge loans | Extended Maturity | Minimum
Loans and Investments
Financing receivable modified, weighted average term increase from modification				2 months
Multifamily | Bridge loans | Extended Maturity | Maximum
Loans and Investments
Financing receivable modified, weighted average term increase from modification				19 months
Multifamily | Bridge loans | Rate of Interest and Extended Maturity
Loans and Investments
Financing receivable, amount modified in period				$ 86,900
Financing receivable modified, weighted average term increase from modification				1 year
Financing receivable modified, pay rate of interest				$ 500
Multifamily | Bridge loans | Rate of Interest and Extended Maturity | Secured Overnight Financing Rate (SOFR)
Loans and Investments
Variable rate, spread				4.25%
Multifamily | Bridge loans | Interest Deferral, Extended Maturity, And Interest Forgiveness
Loans and Investments
Number of financing receivables modified during the period | loan				3
Financing receivable, amount modified in period				$ 241,000
Financing receivable modified, weighted average term increase from modification				1 year
Percentage of deferred interest waived				0.25
Multifamily | Bridge loans | Interest Deferral, Extended Maturity, And Interest Forgiveness | Minimum
Loans and Investments
Percentage of interest foregone				0.0200
Multifamily | Bridge loans | Interest Deferral, Extended Maturity, And Interest Forgiveness | Minimum | Secured Overnight Financing Rate (SOFR)
Loans and Investments
Variable rate, spread				4.00%
Multifamily | Bridge loans | Interest Deferral, Extended Maturity, And Interest Forgiveness | Maximum
Loans and Investments
Percentage of interest foregone				0.0215
Multifamily | Bridge loans | Interest Deferral, Extended Maturity, And Interest Forgiveness | Maximum | Secured Overnight Financing Rate (SOFR)
Loans and Investments
Variable rate, spread				4.30%
Multifamily | Financial Asset, Less Than 60 Days Past Due | Bridge loans
Loans and Investments
Accrued interest receivable related to loans			$ 489,400	$ 956,900			$ 956,900
Number of loans with late and partial payments | loan			12	24			24
Loans now less than 60 days past due			$ 420,300
Multifamily | Financial Asset, Greater Than 60 Days Past Due | Bridge loans
Loans and Investments
Accrued interest receivable related to loans			174,900
Multifamily | Non-performing loans
Loans and Investments
Bridge loans, total UPB			462,207				$ 271,532
Multifamily | Non-performing loans | Greater Than 90 Days Past Due
Loans and Investments
Multifamily bridge loan			462,207	$ 271,532			271,532
Office
Loans and Investments
Provision for credit losses			(49)			(56)
Loans and investments, allowance for credit losses			93	142		$ 8,106	142	$ 8,162
Financing receivable, net of reserve				37,100			37,100
Financing receivable, reserve for credit loss				8,000			8,000
Loan reserve				2,300
Credit risk concentration
Loans and Investments
Multifamily bridge loan			12,249,862	12,615,006			12,615,006
Credit risk concentration | Land
Loans and Investments
Multifamily bridge loan			136,028	136,028			136,028
Credit risk concentration | Multifamily
Loans and Investments
Multifamily bridge loan			10,606,448	11,097,981			11,097,981
Credit risk concentration | Office
Loans and Investments
Multifamily bridge loan			$ 35,410	$ 35,410			$ 35,410
Total Assets | Credit risk concentration
Loans and Investments
Number of loans | loan			31				31
Number of different borrowers | borrower			5				5
Total Assets | Credit risk concentration | Five Borrowers
Loans and Investments
Concentration risk, percentage			11.00%				11.00%
Loans and investments portfolio | Credit risk concentration | New York
Loans and Investments
Concentration risk, percentage			23.00%				24.00%
Loans and investments portfolio | Credit risk concentration | Florida
Loans and Investments
Concentration risk, percentage			17.00%				17.00%

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