Accrued management and incentive fees payable.

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Expenses incurred on insurance agency transaction of affiliate with loan borrower.

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Income received on insurance agency transaction of affiliate with loan borrower.

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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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Securitized debt obligations.

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Special servicing fees.

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Subordinate risk retention securitization.

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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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Number of mortgage loans 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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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, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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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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No definition available.

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

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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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The distributions of earnings to an entity that is affiliated with the reporting entity by means of direct or indirect ownership.

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The cash inflow associated with the amount received from the sale of a business segment or subsidiary or sale of an entity that is related to it but not strictly controlled during the period (for example, an unconsolidated subsidiary, affiliate, joint venture or equity method investment).

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This element represents the cash inflow during the period from the sale of a component of the entity.

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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 Billions	12 Months Ended
	Dec. 31, 2018 USD ($) SecurityLoan shares	Dec. 31, 2017 USD ($) SecurityLoan shares	Dec. 31, 2016 USD ($) shares	Dec. 31, 2018 GBP (£) shares	Jul. 31, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Mar. 31, 2018 EUR (€)	Jun. 30, 2017 USD ($)
Related Party Transaction [Line Items]
Common stock value	$ 1,234,000	$ 1,079,000
Non-cash expenses	$ 28,154,000	$ 24,031,000	$ 21,899,000
Number of Loans \| SecurityLoan	125	110
Loan face amount	$ 14,293,970,000	$ 10,108,226,000
Fully subscribed offering costs					$ 1,000,000,000				$ 474,600,000
CT Legacy Partners [Member]
Related Party Transaction [Line Items]
Total incentive compensation payments			$ 1,094,000
Restricted Class A Common Stock [Member]
Related Party Transaction [Line Items]
Shares held \| shares	1,614,907	1,484,175	1,309,995	1,614,907
Restricted shares, vesting period	3 years
Senior Term Facility [Member]
Related Party Transaction [Line Items]
Loan face amount				£ 303.5				€ 7.3
Senior term facility				£ 148.7				€ 1.0
Minority participation in senior term facility	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	$ 18,600,000	$ 14,300,000
Management fees paid to Manager	70,500,000	53,400,000	$ 57,500,000
Expenses reimbursed to Manager	$ 836,000	$ 621,000	462,000
Number of Loans \| SecurityLoan	9	6
Expenses incurred on insurance agency transaction of affiliate with loan borrower	$ 0	$ 0
Income received on insurance agency transaction of affiliate with loan borrower	0	0
Securitized debt obligations issued, aggregate amount					$ 116,100,000				$ 72,900,000
Total incentive compensation payments	27,900,000	16,200,000	18,000,000
Manager [Member] \| Senior Loan [Member]
Related Party Transaction [Line Items]
Loan face amount			79,000,000				$ 330,000,000
Subordinate risk retention securitization						$ 99,000,000
Manager [Member] \| CT Legacy Partners [Member]
Related Party Transaction [Line Items]
Net sale proceeds			41,300,000
Net sales proceeds, amount received			3,600,000
Manager [Member] \| Incentive Awards Plan [Member] \| CT Legacy Partners [Member]
Related Party Transaction [Line Items]
Total incentive compensation payments	$ 2,800,000
Incentive compensation amount paid			244,000
Manager [Member] \| Unaffiliated Third Party [Member] \| Senior Loan [Member]
Related Party Transaction [Line Items]
Loan face amount						$ 1,000,000,000
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%
Manager [Member] \| Class A Common Stock [Member] \| Restricted Class A Common Stock [Member]
Related Party Transaction [Line Items]
Shares held \| shares	957,874			957,874
Common stock value	$ 31,900,000
Non-cash expenses	$ 13,500,000	11,700,000	9,600,000
Manager [Member] \| Class A Common Stock [Member] \| Restricted Class A Common Stock [Member] \| Stock Incentive Current Plan [Member]
Related Party Transaction [Line Items]
Restricted shares, vesting period	3 years
Affiliates of Manager [Member]
Related Party Transaction [Line Items]
Special servicing fees	$ 0	0
Affiliates of Manager [Member] \| Third-Party Service Provider [Member]
Related Party Transaction [Line Items]
Administrative services expenses incurred	450,000	411,000	385,000
BXMT Advisors Limited Liability Company and Affiliates [Member]
Related Party Transaction [Line Items]
Payments of preferred distributions to affiliate of our Manager	$ 0	$ 0	$ 616,000
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%