Accrued management and incentive fees payable.

No definition available.

Name:	bxmt_AccruedManagementAndIncentiveFeesPayable
Namespace Prefix:	bxmt_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Additional acquisition of loan.

+ References

No definition available.

+ Details

Name:	bxmt_AdditionalAcquisitionOfLoan
Namespace Prefix:	bxmt_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Management agreement initial term expiry date.

+ References

No definition available.

+ Details

Name:	bxmt_ManagementAgreementInitialTermExpiryDate
Namespace Prefix:	bxmt_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Management agreement renewal period.

+ References

No definition available.

+ Details

Name:	bxmt_ManagementAgreementRenewalPeriod
Namespace Prefix:	bxmt_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Management agreement renewal term description.

+ References

No definition available.

+ Details

Name:	bxmt_ManagementAgreementRenewalTermDescription
Namespace Prefix:	bxmt_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of senior notes originated.

+ References

No definition available.

+ Details

Name:	bxmt_NumberOfSeniorNotesOriginated
Namespace Prefix:	bxmt_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

percentage of total secured term loans.

+ References

No definition available.

+ Details

Name:	bxmt_PercentageOfTotalSecuredTermLoans
Namespace Prefix:	bxmt_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Securitized debt obligations.

+ References

No definition available.

+ Details

Name:	bxmt_SecuritizedDebtObligations
Namespace Prefix:	bxmt_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

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.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 12
-Paragraph Column F
-Subsection 29

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 948
-SubTopic 310
-Section S99
-Paragraph 1
-Subparagraph (SX 210.12-29(Column F))
-URI http://asc.fasb.org/extlink&oid=6589523&loc=d3e617274-123014

+ Details

Name:	srt_MortgageLoansOnRealEstateFaceAmountOfMortgages
Namespace Prefix:	srt_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

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.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 850
-SubTopic 10
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39549-107864

+ Details

Name:	us-gaap_AdministrativeFeesExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

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.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(29))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_CommonStockValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

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.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=109500613&loc=SL6031897-161870

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI http://asc.fasb.org/extlink&oid=119400593&loc=d3e572229-122910

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

Name:	us-gaap_DebtInstrumentCarryingAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

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).

+ References

No definition available.

+ Details

Name:	us-gaap_DebtSecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

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

+ References

+ Details

Name:	us-gaap_IncentiveFeeExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

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

+ References

No definition available.

+ Details

Name:	us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

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

+ References

No definition available.

+ Details

Name:	us-gaap_MinorityInterestOwnershipPercentageByParent
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

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).

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 17
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3367-108585

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 850
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39549-107864

+ Details

Name:	us-gaap_PaymentForManagementFee
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

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

+ References

No definition available.

+ Details

Name:	us-gaap_ProfessionalAndContractServicesExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

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.

+ References

No definition available.

+ Details

Name:	us-gaap_RelatedPartyTransactionLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

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.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_SeniorLongTermNotes
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of noncash expense for share-based payment arrangement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3602-108585

+ Details

Name:	us-gaap_ShareBasedCompensation
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

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.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(i)-(ii)
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	us-gaap_UnderlyingAssetClassAxis=bxmt_TwoThousandAndEighteenSingleAssetSecuritizationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=bxmt_SecuredTermLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=us-gaap_RestrictedStockMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_OwnershipAxis=bxmt_SeniorTermFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_LoanPurchaseCommitmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_OwnershipAxis=bxmt_UnaffiliatedThirdPartyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=bxmt_BXMTAdvisorsLimitedLiabilityCompanyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_SeniorLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RealEstatePropertiesAxis=us-gaap_PartiallyOwnedPropertiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementEquityComponentsAxis=us-gaap_CommonStockMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=srt_AffiliatedEntityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionAxis=bxmt_ThirdPartyServicesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=bxmt_BxmtAdvisorsLimitedLiabilityCompanyAndAffiliatesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_TitleOfIndividualAxis=bxmt_ManagerMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=bxmt_BlackstoneAdvisedInvestmentVehiclesOrTheFundsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Transactions with Related Parties - Additional Information (Detail) € in Millions, £ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2019 USD ($) Loans	Jun. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) Loans	Sep. 30, 2018 USD ($)	Sep. 30, 2019 EUR (€) shares	Sep. 30, 2019 USD ($) shares	Jun. 30, 2019 EUR (€)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 GBP (£)	Dec. 31, 2018 USD ($) shares	Jun. 30, 2018 USD ($)	Mar. 31, 2018 EUR (€)	Mar. 31, 2018 USD ($)
Related Party Transaction [Line Items]
Common stock value							$ 1,343,000				$ 1,234,000
Restricted shares, vesting period				3 years
Non-cash expenses				$ 23,276,000	$ 20,488,000
Number of senior notes originated \| Loans	2			2
2018 Single Asset Securitization [Member]
Related Party Transaction [Line Items]
Fully subscribed offering costs			$ 1,000,000,000.0		1,000,000,000.0		$ 993,500,000				$ 1,000,000,000.0
Secured Term Loan [Member]
Related Party Transaction [Line Items]
Debt Securities									$ 55,000,000.0
percentage of total secured term loans								11.00%	11.00%
Restricted Class A Common Stock [Member]
Related Party Transaction [Line Items]
Shares held \| shares						1,220,724	1,220,724				1,614,907
Restricted shares, vesting period				3 years
Senior Term Facility [Member]
Related Party Transaction [Line Items]
Loan face amount						€ 1,600.0		€ 391.3		£ 490.0			€ 7,300.0
Senior term facility						€ 125.0		€ 191.8		£ 240.1			€ 1,000.0
Minority participation in senior term facility						16.00%	16.00%	49.00%	49.00%	49.00%			14.00%	14.00%
Senior Term Facility [Member] \| Loan Purchase Commitments [Member]
Related Party Transaction [Line Items]
Incremental loan acquisition							$ 125,000,000
Unaffiliated Third Party [Member]
Related Party Transaction [Line Items]
Loan face amount							437,400,000
Senior term facility							$ 214,300,000
Minority participation in senior term facility						49.00%	49.00%
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							$ 17,500,000				$ 18,600,000
Management fees paid to Manager	$ 21,000,000.0		22,400,000	$ 59,400,000	52,200,000
Expenses reimbursed to Manager	335,000		167,000	766,000	572,000
Securitized debt obligations issued, aggregate amount			116,100,000		116,100,000
Total incentive compensation payments	3,100,000		6,300,000	17,400,000	22,000,000.0
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
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						606,376	606,376
Common stock value							$ 19,200,000
Non-cash expenses	3,800,000		3,200,000	11,500,000	9,600,000
Affiliates of Manager [Member] \| Third-Party Service Provider [Member]
Related Party Transaction [Line Items]
Administrative services expenses incurred	$ 106,000		$ 90,000	$ 282,000	$ 384,000
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%