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Management agreement initial term expiry date.

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Management agreement renewal period.

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Management agreement renewal term description.

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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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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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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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Aggregate par or stated value of issued nonredeemable common stock (or common stock redeemable solely at the option of the issuer). This item includes treasury stock repurchased by the entity. Note: elements for number of nonredeemable common shares, par value and other disclosure concepts are in another section within stockholders' equity.

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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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Amount of expense for incentive rights held by the managing member or general partner, of limited liability company (LLC) or limited partnership (LP).

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

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The parent entity's interest in net assets of the subsidiary, expressed as a percentage.

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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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The aggregate amount of noncash, equity-based employee remuneration. This may include the value of stock or unit options, amortization of restricted stock or units, and adjustment for officers' compensation. As noncash, this element is an add back when calculating net cash generated by operating activities using the indirect method.

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Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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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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Transactions with Related Parties - Additional Information (Detail) € in Millions, £ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019 USD ($) shares	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) shares	Jun. 30, 2018 USD ($)	Jun. 30, 2019 EUR (€) shares	Mar. 31, 2019 GBP (£)	Dec. 31, 2018 USD ($) shares	Mar. 31, 2018 USD ($)	Mar. 31, 2018 EUR (€)
Related Party Transaction [Line Items]
Common stock value	$ 1,343,000		$ 1,343,000				$ 1,234,000
Restricted shares, vesting period			3 years
Non-cash expenses			$ 15,522,000	$ 13,754,000
Secured Term Loan [Member]
Related Party Transaction [Line Items]
Debt Securities	$ 55,000,000.0		$ 55,000,000.0
percentage of total secured term loans	11.00%		11.00%		11.00%
Restricted Class A Common Stock [Member]
Related Party Transaction [Line Items]
Shares held \| shares	1,511,466		1,511,466		1,511,466		1,614,907
Restricted shares, vesting period			3 years
Senior Term Facility [Member]
Related Party Transaction [Line Items]
Loan face amount					€ 391.3	£ 490.0			€ 7,300.0
Senior term facility					€ 191.8	£ 240.1			€ 1,000.0
Minority participation in senior term facility	49.00%		49.00%		49.00%	49.00%		14.00%	14.00%
Manager [Member]
Related Party Transaction [Line Items]
Management Agreement expiration date			Dec. 19, 2019
Management Agreement renewal term, description			the current term of which expires on December 19, 2019, and will be automatically renewed for a one-year term upon such date and each anniversary thereafter unless earlier terminated.
Management Agreement renewal term, period			1 year
Accrued management and incentive fees payable	$ 21,000,000.0		$ 21,000,000.0				$ 18,600,000
Management fees paid to Manager	19,800,000	$ 15,500,000	38,400,000	29,800,000
Expenses reimbursed to Manager	242,000	215,000	430,000	405,000
Total incentive compensation payments	$ 7,700,000	11,300,000	$ 14,400,000	15,700,000
Manager [Member] \| Senior Loan [Member]
Related Party Transaction [Line Items]
Loan face amount								$ 330,000,000.0
Manager [Member] \| Unaffiliated Third Party [Member] \| Senior Loan [Member]
Related Party Transaction [Line Items]
Loan face amount		$ 1,000,000,000.0		$ 1,000,000,000.0
Manager [Member] \| Unaffiliated Third Party [Member] \| Partially Owned Properties [Member] \| Senior Loan [Member]
Related Party Transaction [Line Items]
Ownership interest in joint venture		50.00%		50.00%
Manager [Member] \| Class A Common Stock [Member] \| Restricted Class A Common Stock [Member]
Related Party Transaction [Line Items]
Shares held \| shares	723,542		723,542		723,542
Common stock value	$ 22,900,000		$ 22,900,000
Non-cash expenses	3,900,000	$ 3,200,000	7,700,000	$ 6,400,000
Affiliates of Manager [Member] \| Third-Party Service Provider [Member]
Related Party Transaction [Line Items]
Administrative services expenses incurred	90,000	$ 153,000	$ 176,000	$ 293,000
BXMT Advisors Limited Liability Company and Affiliates [Member] \| Secured Term Loan [Member] \| Manager [Member]
Related Party Transaction [Line Items]
Total incentive compensation payments	$ 500,000
Blackstone-Advised Investment Vehicles, or the Funds [Member] \| Senior Term Facility [Member]
Related Party Transaction [Line Items]
Ownership interest in joint venture								51.00%	51.00%