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Management agreement initial term expiry date.

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Management agreement renewal term description.

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Percentage of senior loan borrowed over mortgage loans on real estate face amount of mortgages.

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percentage of total secured term loans.

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Amount of contractual principal due at origination of mortgage loan within investment in mortgage loan on real estate by entity with substantial portion of business acquiring and holding investment real estate or interest in real estate.

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Amount of expense for administrative services provided to the limited liability company (LLC) or limited partnership (LP) by the managing member or general partner, affiliate of managing member or general partner, or affiliate of LLC or LP, for example, but not limited to, salaries, rent, or overhead costs.

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Costs of sales and operating expenses for the period incurred from transactions with related parties.

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Net increase or decrease in the carrying amount of the debt instrument for the period.

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Amount of investment in debt security measured at amortized cost (held-to-maturity), investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale) and investment in debt security measured at fair value with change in fair value recognized in net income (trading).

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The equity interest of noncontrolling shareholders, partners or other equity holders in consolidated entity.

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Amount paid to managing member or general partner for management of the day-to-day business functions of the limited liability company (LLC) or limited partnership (LP).

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Professional and contract service expense includes cost reimbursements for support services related to contracted projects, outsourced management, technical and staff support.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Carrying value as of the balance sheet date of Notes with the highest claim on the assets of the issuer in case of bankruptcy or liquidation (with maturities initially due after one year or beyond the operating cycle if longer), excluding current portion. Senior note holders are paid off in full before any payments are made to junior note holders.

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Amount of noncash expense for share-based payment arrangement.

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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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Number of shares issued in lieu of cash for services contributed to the entity. Number of shares includes, but is not limited to, shares issued for services contributed by vendors and founders.

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Transactions with Related Parties - Additional Information (Detail) € in Millions, kr in Billions	3 Months Ended			6 Months Ended
	Jun. 30, 2021 USD ($) shares	Jun. 30, 2020 USD ($) shares	Mar. 31, 2020 USD ($)	Jun. 30, 2021 USD ($) shares	Jun. 30, 2020 USD ($)	Mar. 31, 2021 USD ($)	Mar. 31, 2021 SEK (kr)	Dec. 31, 2020 USD ($) shares	Dec. 31, 2019 USD ($)	Dec. 31, 2019 EUR (€)	Sep. 30, 2019 EUR (€)	Jun. 30, 2019 USD ($)
Related Party Transaction [Line Items]
Non-cash expenses				$ 16,105,000	$ 17,329,000
Loan face amount	$ 17,529,542,000			$ 17,529,542,000				$ 16,652,824,000
Restricted Class A Common Stock [Member]
Related Party Transaction [Line Items]
Shares held \| shares	1,424,811			1,424,811				1,627,890
Senior Term Facility [Member]
Related Party Transaction [Line Items]
Percentage of senior loan borrowed	22.00%			22.00%
Increase of senior term loan facility	$ 100,000,000.0
Manager [Member]
Related Party Transaction [Line Items]
Management Agreement expiration date				Dec. 19, 2021
Management Agreement renewal term, description				the current term of which expires on December 19, 2021, and will be automatically renewed for a one-year term upon such date and each anniversary thereafter unless earlier terminated.
Accrued management and incentive fees payable	21,500,000			$ 21,500,000				$ 19,200,000
Management fees paid to Manager	19,200,000	$ 19,300,000		38,400,000	39,400,000
Expenses reimbursed to Manager	$ 145,000	$ 205,000		$ 185,000	423,000
Manager [Member] \| Class A Common Stock [Member]
Related Party Transaction [Line Items]
Stock Issued During Period, Shares, Issued for Services \| shares		840,696
Management fees paid to Manager			$ 19,300,000
Manager [Member] \| Class A Common Stock [Member] \| Restricted Class A Common Stock [Member]
Related Party Transaction [Line Items]
Shares held \| shares	699,961			699,961
Non-cash expenses	$ 4,100,000	$ 4,200,000		$ 8,100,000
Manager [Member] \| Class A Common Stock [Member] \| Restricted Class A Common Stock [Member] \| Manager [Member]
Related Party Transaction [Line Items]
Non-cash expenses				22,300,000	8,500,000
Affiliates of Manager [Member] \| Third-Party Service Provider [Member]
Related Party Transaction [Line Items]
Administrative services expenses incurred	92,000	138,000		192,000	271,000
BXMT Advisors Limited Liability Company and Affiliates [Member]
Related Party Transaction [Line Items]
Accrued management and incentive fees payable								$ 19,200,000
BXMT Advisors Limited Liability Company and Affiliates [Member] \| B Two Term Loan [Member]
Related Party Transaction [Line Items]
Total incentive compensation payments				350,000
BXMT Advisors Limited Liability Company and Affiliates [Member] \| B One Term Loan [Member]
Related Party Transaction [Line Items]
Total incentive compensation payments				950,000
Blackstone-Advised Investment Vehicles, or the Funds [Member] \| B Two Term Loan [Member]
Related Party Transaction [Line Items]
Debt Securities	$ 20,000,000.0	$ 20,000,000.0		$ 20,000,000.0	$ 20,000,000.0
percentage of total secured term loans	5.00%	5.00%		5.00%	5.00%
Blackstone-Advised Investment Vehicles, or the Funds [Member] \| B One Term Loan [Member]
Related Party Transaction [Line Items]
Debt Securities						$ 65,500,000			$ 65,500,000			$ 65,500,000
percentage of total secured term loans						7.00%	7.00%		7.00%	7.00%		7.00%
Blackstone-Advised Investment Vehicles, or the Funds [Member] \| Senior Term Facility [Member]
Related Party Transaction [Line Items]
Loan face amount	$ 491,000,000.0		421,500,000	$ 491,000,000.0			kr 10.2			€ 1,600.0	€ 1,600.0
Senior term facility	$ 50,000,000.0		$ 140,000,000.0	$ 50,000,000.0			kr 5.0			€ 250.0	€ 250.0
Minority participation in senior term facility			33.00%			49.00%	49.00%		16.00%	16.00%	16.00%