Transactions with Related Parties (Detail) £ in Millions	3 Months Ended
	Mar. 31, 2024 USD ($) shares	Mar. 31, 2023 USD ($)	Dec. 31, 2023 USD ($) shares	Mar. 31, 2019 GBP (£)
Related Party Transaction [Line Items]
Non-cash expenses	$ 8,112,000	$ 7,655,000
Common stock, shares outstanding (in shares) | shares	173,582,305		173,209,933
Principal collections, sales proceeds, and cost-recovery proceeds from loans receivable	$ 637,242,000	562,072,000
Senior Secured Notes Due 2027 | Senior Secured Notes
Related Party Transaction [Line Items]
Repurchase of aggregate principal amount	$ 26,200,000	0
Senior Term Facility
Related Party Transaction [Line Items]
Senior term facility | £				£ 240.1
Face amount of loans | £				£ 490.0
Restricted Class A Common Stock
Related Party Transaction [Line Items]
Shares held (in shares) | shares	2,345,996		2,180,181
Restricted shares, vesting period (in years)	3 years
Related Party
Related Party Transaction [Line Items]
Principal collections, sales proceeds, and cost-recovery proceeds from loans receivable	$ 98,600,000
Related Party | Class A Common Stock
Related Party Transaction [Line Items]
Common stock, shares outstanding (in shares) | shares	12,848,779
Percentage of stock (in shares)	7.40%
Related Party | Manager
Related Party Transaction [Line Items]
Renewal term (in years)	1 year
Management fees paid to Manager	$ 26,300,000	33,800,000
Expenses	221,000	382,000
Related Party | BXMT Advisors Limited Liability Company and Affiliates
Related Party Transaction [Line Items]
Accrued management fee payable	18,900,000
Accrued incentive fees	0
Accrued management and incentive fees payable			$ 26,300,000
Related Party | CTIMCO | Collateralized Loan Obligations
Related Party Transaction [Line Items]
Special servicing fees earned	0	0
Related Party | EQ Management, LLC
Related Party Transaction [Line Items]
Expenses	0
Related Party | BTIG, LLC
Related Party Transaction [Line Items]
Fees on broker	40,000
Related Party | Revantage Corporate Services, LLC And Revantage Global Services Europe S.à r.l.
Related Party Transaction [Line Items]
Administrative services expenses incurred	251,000	214,000
Related Party | Gryphon Mutual Property America IC ("Gryphon")
Related Party Transaction [Line Items]
Insurance costs, inclusive of premiums, capital surplus contributions, taxes and pro-rata share of other expenses	$ 109,000
Related Party | Restricted Class A Common Stock | Class A Common Stock | Manager
Related Party Transaction [Line Items]
Shares held (in shares) | shares	1,207,623
Non-cash expenses	$ 4,300,000	3,900,000
Related Party | Restricted Class A Common Stock | Class A Common Stock | Manager | Manager
Related Party Transaction [Line Items]
Non-cash expenses	$ 29,300,000
Restricted shares, vesting period (in years)	3 years
Affiliates of Manager
Related Party Transaction [Line Items]
Percentage of stock (in shares)	4.40%
Administrative services expenses incurred	$ 24,000	$ 24,000
Affiliates of Manager | Class A Common Stock
Related Party Transaction [Line Items]
Common stock, shares outstanding (in shares) | shares	7,582,044
Affiliates of Manager | Gryphon Mutual Property America IC ("Gryphon")
Related Party Transaction [Line Items]
Management fees paid to Manager	$ 2,000
Director | Deferred Stock Units
Related Party Transaction [Line Items]
Common stock, shares outstanding (in shares) | shares	370,173