Transactions with Related Parties (Detail) $ in Thousands, £ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2024 USD ($) shares	Jun. 30, 2024 GBP (£)	Mar. 31, 2024 USD ($)	Jun. 30, 2023 USD ($)	Jun. 30, 2024 USD ($) shares	Jun. 30, 2023 USD ($)	Jun. 30, 2024 GBP (£) shares	Dec. 31, 2023 USD ($) shares	Mar. 31, 2019 GBP (£)
Related Party Transaction [Line Items]
Accrued management and incentive fees payable	$ 18,700				$ 18,700			$ 26,300
Non-cash expenses	$ 7,962			$ 7,665	$ 16,074	$ 15,320
Common stock, shares outstanding (in shares) | shares	173,619,498				173,619,498		173,619,498	173,209,933
Expenses	$ 18,726			32,815	$ 37,653	63,865
Professional services	$ 3,817			3,291	7,957	6,570
Repayment of loan participations					235,960	0
Principal collections, sales proceeds, and cost-recovery proceeds from loans receivable					1,413,348	1,491,158
Senior Secured Notes
Related Party Transaction [Line Items]
Repurchase of aggregate principal amount				0	$ 26,200	0
Restricted Class A Common Stock
Related Party Transaction [Line Items]
Shares held (in shares) | shares	1,938,160				1,938,160		1,938,160	2,180,181
Restricted shares, vesting period (in years)					3 years
Related Party
Related Party Transaction [Line Items]
Common stock, shares outstanding (in shares) | shares	12,226,812				12,226,812		12,226,812
Percentage of stock (in shares)	7.00%				7.00%		7.00%
Expenses	$ 18,700			18,600	$ 37,700	37,200
Principal collections, sales proceeds, and cost-recovery proceeds from loans receivable			$ 98,600
Face amount of loans | £							£ 490.0		£ 240.1
Related Party | Junior Loan Participation
Related Party Transaction [Line Items]
Repayment of loan participations | £		£ 46.5
Related Party | Senior Participations
Related Party Transaction [Line Items]
Repayment of loan participations | £		£ 186.0
Related Party | Manager
Related Party Transaction [Line Items]
Renewal term (in years)					1 year
Management fees paid to Manager	18,900			31,100	$ 45,300	64,900
Expenses	829			1,500	1,100	1,900
Related Party | BXMT Advisors Limited Liability Company and Affiliates
Related Party Transaction [Line Items]
Management fee payable	18,700				18,700
Incentive fee payable	0				0
Accrued management and incentive fees payable								$ 26,300
Related Party | EQ Management, LLC
Related Party Transaction [Line Items]
Expenses	44			0	44	0
Related Party | Revantage Corporate Services, LLC And Revantage Global Services Europe S.à r.l.
Related Party Transaction [Line Items]
Professional services	$ 309			219	560	433
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
Related Party | BTIG, LLC
Related Party Transaction [Line Items]
Fees on broker					40
Related Party | BTIG, LLC | Senior Secured Notes | Senior Secured Notes Due 2027
Related Party Transaction [Line Items]
Repurchase of aggregate principal amount					$ 26,200
Related Party | Restricted Class A Common Stock | Manager
Related Party Transaction [Line Items]
Shares held (in shares) | shares	989,709				989,709		989,709
Aggregate grant fair value					$ 23,800
Restricted shares, vesting period (in years)					3 years
Non-cash expenses	$ 4,200			3,900	$ 8,500	7,800
Subsidiary of Common Parent
Related Party Transaction [Line Items]
Common stock, shares outstanding (in shares) | shares	7,582,044				7,582,044		7,582,044
Percentage of stock (in shares)	4.40%				4.40%		4.40%
Director | Deferred Stock Units
Related Party Transaction [Line Items]
Shares held (in shares) | shares	389,113				389,113		389,113
Affiliates of Manager
Related Party Transaction [Line Items]
Professional services	$ 24			$ 24	$ 48	$ 48
Affiliates of Manager | Gryphon Mutual Property America IC ("Gryphon")
Related Party Transaction [Line Items]
Management fees paid to Manager					$ 2