Collateralized Loan Obligation - Schedule of Assets and Liabilities Included in Consolidated Balance Sheet (Details) - USD ($) $ in Thousands		Sep. 30, 2021	Dec. 31, 2020
Assets and Associated Liabilities of Transfers Accounted for as Secured Borrowings [Line Items]
Commercial real estate loans, held-for-investment		$ 5,442,734	$ 4,844,534
Less: Allowance for credit losses		(58,807)	(59,801)
Commercial real estate loans, held-for-investment, net		5,383,927	4,784,733
Accrued interest receivable		13,924	15,412
Other assets	[1]	13,107	20,984
Total Assets		5,753,498	4,965,612
Collateralized loan obligations, net		1,086,900	810,000
Accrued interest payable		7,495	5,381
Accounts payable, accrued expenses and other liabilities	[2]	3,065	4,823
Total Liabilities		4,516,321	3,920,206
Unamortized debt issuance costs			5,600
Collateralized Loan Obligations
Assets and Associated Liabilities of Transfers Accounted for as Secured Borrowings [Line Items]
Commercial real estate loans, held-for-investment, net		1,300,000	1,000,000
Collateralized Loan Obligations | Primary Beneficiary
Assets and Associated Liabilities of Transfers Accounted for as Secured Borrowings [Line Items]
Commercial real estate loans, held-for-investment		1,300,000	1,000,483
Less: Allowance for credit losses		(1,240)	(2,669)
Commercial real estate loans, held-for-investment, net		1,298,760	997,814
Accrued interest receivable		3,481	3,075
Other assets		5	5
Total Assets		1,302,246	1,000,894
Collateralized loan obligations, net		1,086,900	810,000
Accrued interest payable		783	668
Accounts payable, accrued expenses and other liabilities		0	72
Total Liabilities		1,087,683	$ 810,740
Collateralized Loan Obligations | Primary Beneficiary | KREF 2021-FL2
Assets and Associated Liabilities of Transfers Accounted for as Secured Borrowings [Line Items]
Unamortized debt issuance costs		$ 8,300

[1] Includes $8.2 million and $15.9 million of loan repayment proceeds held by servicer and receivable by KREF as of September 30, 2021 and December 31, 2020, respectively.

[2] Includes $0.9 million of expected loss reserve for unfunded loan commitments as of September 30, 2021 and December 31, 2020.