Debt instrument covenant fixed charge coverage ratio.

No definition available.

Name:	bxmt_DebtInstrumentCovenantFixedChargeCoverageRatio1
Namespace Prefix:	bxmt_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Debt Instrument Covenant Liquidity

+ References

No definition available.

+ Details

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

- Definition

Debt Instrument Covenant, Minimum Percentage Of Cash Liquidity On Recourse Indebtedness

+ References

No definition available.

+ Details

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

- Definition

Debt Instrument Covenant, Minimum Percentage Of Tangible Net Worth On Net Cash Proceed Of Equity Issuances

+ References

No definition available.

+ Details

Name:	bxmt_DebtInstrumentCovenantMinimumPercentageOfTangibleNetWorthOnNetCashProceedOfEquityIssuances
Namespace Prefix:	bxmt_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Debt Instrument Covenant Requirement On Consolidated Tangible Net Worth Minimum

+ References

No definition available.

+ Details

Name:	bxmt_DebtInstrumentCovenantRequirementOnConsolidatedTangibleNetWorthMinimum
Namespace Prefix:	bxmt_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Debt Instrument Revolving Repurchase Facilities Advance Rate

+ References

No definition available.

+ Details

Name:	bxmt_DebtInstrumentRevolvingRepurchaseFacilitiesAdvanceRate
Namespace Prefix:	bxmt_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Maximum time period that a specific collateral loan can be pledged to the facility.

+ References

No definition available.

+ Details

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

- Definition

Number of new credit facility.

+ References

No definition available.

+ Details

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

- Definition

Ratio Of Indebtedness To Asset Value

+ References

No definition available.

+ Details

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

- Definition

Secured Debt Repurchase Agreements Gross

+ References

No definition available.

+ Details

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

- Definition

The amount of amortization of deferred charges applied against earnings during the period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.3)
-URI http://asc.fasb.org/extlink&oid=115205541&loc=SL114868664-224227

+ Details

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

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
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_DebtInstrumentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of the cost of borrowed funds accounted for as interest expense.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=6450988&loc=d3e26243-108391

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28555-108399

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 22
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=115929826&loc=d3e8736-108599

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (210.5-03(11))
-URI http://asc.fasb.org/extlink&oid=115205541&loc=SL114868664-224227

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04.9)
-URI http://asc.fasb.org/extlink&oid=114873765&loc=SL114874048-224260

+ Details

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

- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

+ 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

Reference 2: 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

Reference 3: 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=6879938&loc=d3e572229-122910

+ Details

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

- Definition

Average amount borrowed under the credit facility during the period.

+ References

No definition available.

+ Details

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

- Definition

Amount of increase (decrease) of the credit facility.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(f))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

+ Details

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

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

+ Details

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

- Definition

Carrying amount as of the balance sheet date of loans that have been pledged as collateral for borrowings. If the secured party has the right by contract or custom to sell or re-pledge the collateral, then the debtor reclassifies that asset and report that asset in its statement of financial position separately from other assets not so encumbered.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=66007379&loc=d3e113888-111728

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5086-111524

+ Details

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

- Definition

Weighted average maturity (weighted by amount of the agreement) of repurchase agreements with the counterparty, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(m)(1)(i)(B)(iii))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

+ Details

Name:	us-gaap_RepurchaseAgreementCounterpartyWeightedAverageMaturityOfAgreements1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Carrying value as of the balance sheet date, including the current and noncurrent portions, of collateralized debt obligations (with maturities initially due after one year or beyond the operating cycle, if longer). Such obligations include mortgage loans, chattel loans, and any other borrowings secured by assets of the borrower.

+ References

Reference 1: 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(a)(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 2: 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

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

+ Details

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

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_RevolvingCreditFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CounterpartyNameAxis=bxmt_BarclaysMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=bxmt_CreditFacilitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=bxmt_NewCreditFacilitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=bxmt_AssetSpecificFinancingsMember
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_VariableRateAxis=us-gaap_LondonInterbankOfferedRateLIBORMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AssetsSoldUnderAgreementsToRepurchaseAxis=bxmt_WeightedAverageCashCouponMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AssetsSoldUnderAgreementsToRepurchaseAxis=bxmt_WeightedAverageAllInCostOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Secured Debt Agreements, Net - Additional Information (Detail) € in Millions	3 Months Ended		9 Months Ended				12 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) Facility	Sep. 30, 2018 GBP (£) Facility	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2018 EUR (€)	Sep. 30, 2018 GBP (£)	Dec. 31, 2017 GBP (£)
Debt Instrument [Line Items]
Aggregate borrowings	$ 5,409,815,000		$ 5,409,815,000
Basis spread on debt obligation, in percent			1.77%	1.77%
Weighted-average advance rate			79.20%	79.20%
Secured debt agreements borrowings outstanding					$ 38,456,000		$ 38,456,000
Maximum Facility Size	651,600,000		$ 651,600,000		675,700,000		675,700,000		£ 500,000,000	£ 500,000,000
Interest expense	97,955,000	$ 67,891,000	255,677,000			$ 168,917,000
Secured debt agreements borrowings outstanding	6,862,256,000		6,862,256,000		5,290,536,000		5,290,536,000
Covenants, minimum tangible net worth	$ 2,400,000,000		$ 2,400,000,000
Covenants, percentage of tangible assets on cash proceeds from equity issuances			75.00%	75.00%
Covenants, percentage of recourse indebtedness	5.00%		5.00%					5.00%	5.00%
Maximum [Member]
Debt Instrument [Line Items]
Covenants, indebtedness to total assets, in percent	83.33%		83.33%					83.33%	83.33%
Minimum [Member]
Debt Instrument [Line Items]
Covenants, EBITDA to fixed charges, in percent			140.00%	140.00%
Covenants, minimum cash liquidity amount	$ 10,000,000		$ 10,000,000
Asset-Specific Financings [Member]
Debt Instrument [Line Items]
Weighted-average outstanding balance			1,300,000,000		522,500,000
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Weighted-average outstanding balance			37,800,000		40,700,000
Interest expense			2,600,000		2,600,000
Amortization of deferred fees and expenses			825,000		703,000
Secured debt agreements borrowings outstanding	0		0		0		0
Revolving Credit Facility [Member] \| Barclays [Member]
Debt Instrument [Line Items]
Maximum Facility Size	250,000,000		$ 250,000,000
First mortgage bridge to term financing maximum period			6 months	6 months
Credit Facilities [Member]
Debt Instrument [Line Items]
Additional credit capacity \| £				£ 250,000,000
Weighted-average outstanding balance			$ 4,400,000,000		4,200,000,000
Aggregate borrowings	4,850,911,000		$ 4,850,911,000		4,068,249,000		$ 4,068,249,000
Weighted-average advance rate			79.20%	79.20%			78.70%
Weighted-average initial maturity			1 year 6 months	1 year 6 months			1 year 6 months
Maximum Facility Size	8,315,710,000		$ 8,315,710,000		7,155,975,000		$ 7,155,975,000
Secured debt agreements borrowings outstanding	4,850,911,000		4,850,911,000		$ 4,068,249,000		$ 4,068,249,000
Credit Facilities [Member] \| Barclays [Member]
Debt Instrument [Line Items]
Aggregate borrowings	617,360,000		617,360,000
Maximum Facility Size	$ 1,000,000,000		$ 1,000,000,000
New Credit Facilities [Member]
Debt Instrument [Line Items]
Number of new credit facility \| Facility			1	1
Additional credit capacity \| £				£ 1,000,000,000
Asset-Specific Financings [Member]
Debt Instrument [Line Items]
Secured debt agreements borrowings outstanding \| €								€ 800.0
Asset-Specific Financings [Member] \| Senior Loan [Member]
Debt Instrument [Line Items]
Secured loan \| €								€ 1,000.0
LIBOR [Member] \| Credit Facilities [Member] \| Weighted-Average Cash Coupon [Member]
Debt Instrument [Line Items]
Basis spread on debt obligation, in percent			1.77%	1.77%			1.90%
LIBOR [Member] \| Credit Facilities [Member] \| Weighted-Average All-in Cost of Credit [Member]
Debt Instrument [Line Items]
Basis spread on debt obligation, in percent			1.98%	1.98%			2.12%