Debt Instrument Revolving Repurchase Facilities Advance Rate

No definition available.

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

- Definition

Debt Instrument Revolving Repurchase Facilities Guarantee Rate

+ References

No definition available.

+ Details

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

- Definition

Debt Instrument Revolving Repurchase Facilities Margin Call Description

+ References

No definition available.

+ Details

Name:	bxmt_DebtInstrumentRevolvingRepurchaseFacilitiesMarginCallDescription
Namespace Prefix:	bxmt_
Data Type:	xbrli:stringItemType
Balance Type:	na
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

The reference rate for the variable rate of the debt instrument, such as LIBOR or the US Treasury rate and the maturity of the reference rate used, such as three months or six months LIBOR.

+ References

No definition available.

+ Details

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

- Definition

Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(2))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)(2)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ Details

Name:	us-gaap_DebtInstrumentMaturityDate
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Description of the maturity date of the debt instrument including whether the debt matures serially and, if so, a brief description of the serial maturities.

+ References

+ Details

Name:	us-gaap_DebtInstrumentMaturityDateDescription
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
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=120391452&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=120398452&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=120400017&loc=d3e572229-122910

+ Details

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

- Definition

Amount of current borrowing capacity under the credit facility considering any current restrictions on the amount that could be borrowed (for example, borrowings may be limited by the amount of current assets), but without considering any amounts currently outstanding under the facility.

+ References

+ Details

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

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

Secured Debt Agreements, Net - Summary of Key Terms of Credit Facilities (Detail) € in Thousands, £ in Thousands, kr in Thousands, $ in Thousands, $ in Thousands, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Mar. 31, 2021 EUR (€)	Mar. 31, 2021 GBP (£)	Mar. 31, 2021 SEK (kr)	Mar. 31, 2021 AUD ($)	Mar. 31, 2021 CAD ($)
Line of Credit Facility [Line Items]
Potential Borrowings	$ 8,979,644	$ 8,726,028
Outstanding Borrowings	$ 8,142,728	7,896,863
Average Rate	100.00%
Deutsche Bank [Member]
Line of Credit Facility [Line Items]
Guarantee	25.00%
Outstanding Borrowings	$ 845,400
Wells Fargo [Member]
Line of Credit Facility [Line Items]
Guarantee	25.00%
Outstanding Borrowings	$ 136,800	182,500
Bank of America [Member]
Line of Credit Facility [Line Items]
Term/Maturity	May 21, 2024
MetLife [Member]
Line of Credit Facility [Line Items]
Term/Maturity	Sep. 23, 2025
JP Morgan [Member]
Line of Credit Facility [Line Items]
Term/Maturity	Jan. 07, 2024
Morgan Stanley [Member]
Line of Credit Facility [Line Items]
Term/Maturity	Sep. 29, 2025
Goldman Sachs -Multi. JV [Member]
Line of Credit Facility [Line Items]
Term/Maturity	Jul. 12, 2022
Bank of America -Multi. JV [Member]
Line of Credit Facility [Line Items]
Term/Maturity	Jul. 19, 2023
Credit Facilities [Member]
Line of Credit Facility [Line Items]
Potential Borrowings	$ 8,979,644
Outstanding Borrowings	$ 8,142,728	$ 7,900,000
Spread	INDEX + 1.65%
Average Rate	73.10%	72.80%
Basis spread on debt obligation, in percent	1.65%	1.63%
Credit Facilities [Member] \| USD LIBOR
Line of Credit Facility [Line Items]
Potential Borrowings	$ 4,940,277
Outstanding Borrowings	$ 4,110,711
Spread	L + 1.68%
Average Rate	73.20%
Basis spread on debt obligation, in percent	1.68%
Credit Facilities [Member] \| EUR LIBOR
Line of Credit Facility [Line Items]
Potential Borrowings \| €			€ 2,029,930
Outstanding Borrowings \| €			€ 2,023,664
Spread	E + 1.44%
Average Rate	72.80%
Basis spread on debt obligation, in percent	1.44%
Credit Facilities [Member] \| GBP LIBOR
Line of Credit Facility [Line Items]
Potential Borrowings \| £				£ 827,379
Outstanding Borrowings \| £				£ 827,379
Spread	L + 1.95%
Average Rate	71.60%
Basis spread on debt obligation, in percent	1.95%
Credit Facilities [Member] \| BBSY
Line of Credit Facility [Line Items]
Potential Borrowings						$ 236,187
Outstanding Borrowings						$ 236,187
Spread	BBSY + 1.91%
Average Rate	72.50%
Basis spread on debt obligation, in percent	1.91%
Credit Facilities [Member] \| CDOR
Line of Credit Facility [Line Items]
Potential Borrowings							$ 63,334
Outstanding Borrowings							$ 63,334
Spread	CDOR + 1.78%
Average Rate	75.80%
Basis spread on debt obligation, in percent	1.78%
Credit Facilities [Member] \| STIBOR
Line of Credit Facility [Line Items]
Potential Borrowings \| kr					kr 2,514,364
Outstanding Borrowings \| kr					kr 2,514,364
Spread	STIBOR + 1.60%
Average Rate	80.00%
Basis spread on debt obligation, in percent	1.60%
Credit Facilities [Member] \| Barclays [Member]
Line of Credit Facility [Line Items]
Guarantee	25.00%
Margin Call	Collateral marks only
Term/Maturity	Term matched
Potential Borrowings	$ 1,679,748	$ 1,443,251
Outstanding Borrowings	$ 1,523,419	1,249,415
Credit Facilities [Member] \| Deutsche Bank [Member]
Line of Credit Facility [Line Items]
Guarantee	67.00%
Margin Call	Collateral marks only
Term/Maturity	Term matched
Potential Borrowings	$ 1,604,238	1,892,211
Outstanding Borrowings	$ 1,498,113	1,847,211
Credit Facilities [Member] \| Wells Fargo [Member]
Line of Credit Facility [Line Items]
Guarantee	25.00%
Margin Call	Collateral marks only
Term/Maturity	Term matched
Potential Borrowings	$ 1,424,351	1,241,357
Outstanding Borrowings	$ 1,190,799	956,780
Credit Facilities [Member] \| Citibank [Member]
Line of Credit Facility [Line Items]
Guarantee	25.00%
Margin Call	Collateral marks only
Term/Maturity	Term matched
Potential Borrowings	$ 981,763	927,531
Outstanding Borrowings	$ 815,675	779,139
Credit Facilities [Member] \| Goldman Sachs [Member]
Line of Credit Facility [Line Items]
Guarantee	25.00%
Margin Call	Collateral marks only
Term/Maturity	Term matched
Potential Borrowings	$ 598,435	615,411
Outstanding Borrowings	$ 598,435	615,411
Credit Facilities [Member] \| Bank of America [Member]
Line of Credit Facility [Line Items]
Guarantee	50.00%
Margin Call	Collateral marks only
Potential Borrowings	$ 468,061	473,678
Outstanding Borrowings	$ 468,061	473,678
Credit Facilities [Member] \| MetLife [Member]
Line of Credit Facility [Line Items]
Guarantee	72.00%
Margin Call	Collateral marks only
Potential Borrowings	$ 284,900	276,605
Outstanding Borrowings	$ 284,900	276,605
Credit Facilities [Member] \| JP Morgan [Member]
Line of Credit Facility [Line Items]
Guarantee	43.00%
Margin Call	Collateral marks only
Potential Borrowings	$ 453,589	449,449
Outstanding Borrowings	$ 409,236	422,096
Credit Facilities [Member] \| Morgan Stanley [Member]
Line of Credit Facility [Line Items]
Guarantee	25.00%
Margin Call	Collateral marks only
Potential Borrowings	$ 531,877	528,846
Outstanding Borrowings	$ 404,877	401,846
Credit Facilities [Member] \| Santander [Member]
Line of Credit Facility [Line Items]
Guarantee	50.00%
Margin Call	Collateral marks only
Term/Maturity	Term matched
Potential Borrowings	$ 259,590	269,501
Outstanding Borrowings	$ 259,590	269,501
Credit Facilities [Member] \| Société Générale [Member]
Line of Credit Facility [Line Items]
Guarantee	25.00%
Margin Call	Collateral marks only
Term/Maturity	Term matched
Potential Borrowings	$ 240,338	237,822
Outstanding Borrowings	$ 240,338	237,822
Credit Facilities [Member] \| US Bank -Multi. JV [Member]
Line of Credit Facility [Line Items]
Guarantee	25.00%
Margin Call	Collateral marks only
Term/Maturity	Term matched
Potential Borrowings	$ 234,864	184,802
Outstanding Borrowings	$ 231,395	181,795
Credit Facilities [Member] \| Goldman Sachs -Multi. JV [Member]
Line of Credit Facility [Line Items]
Guarantee	25.00%
Margin Call	Collateral marks only
Potential Borrowings	$ 217,890	167,964
Outstanding Borrowings	$ 217,890	167,964
Credit Facilities [Member] \| Bank of America -Multi. JV [Member]
Line of Credit Facility [Line Items]
Guarantee	43.00%
Margin Call	Collateral marks only
Potential Borrowings		17,600
Outstanding Borrowings		$ 17,600