Common Stock Purchased

No definition available.

Name:	bxmt_CommonStockPurchased
Namespace Prefix:	bxmt_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Purchase price of loan portfolio.

+ References

No definition available.

+ Details

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

- Definition

The maximum number of total shares permitted to be issued by an entity's charter and bylaws.

+ References

No definition available.

+ Details

Name:	bxmt_StockSharesAuthorized
Namespace Prefix:	bxmt_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Underwriter Common Stock Overallotment Purchase Option Period

+ References

No definition available.

+ Details

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

- Definition

Unfunded commitments assumed.

+ References

No definition available.

+ Details

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

- Definition

The maximum number of common shares permitted to be issued by an entity's charter and bylaws.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-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

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 30

-Article 5

+ Details

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

- Definition

The cash outflow for cost incurred directly with the issuance of an equity security.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Financing Activities

-URI http://asc.fasb.org/extlink&oid=6513228

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 15

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3291-108585

+ Details

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

- Definition

This is the result of the principal amount outstanding of total loans managed or securitized less securitized loans and loans that are in the process of being securitized.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 860

-SubTopic 20

-Section 50

-Paragraph 4

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=13989330&loc=d3e107314-111719

+ Details

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

- Definition

Cash received on stock transaction after deduction of issuance costs.

+ References

No definition available.

+ Details

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

- Definition

The number of shares issued or sold by the subsidiary or equity method investee per stock transaction.

+ References

No definition available.

+ Details

Name:	us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Per share amount received by subsidiary or equity investee for each share of common stock issued or sold in the stock transaction.

+ References

No definition available.

+ Details

Name:	us-gaap_SaleOfStockPricePerShare
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- 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://www.xbrl.org/2003/role/presentationRef

-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 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 16

-Article 9

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 20, 22

-Article 5

Reference 4: http://www.xbrl.org/2003/role/presentationRef

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

- Details

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

- Details

Name:	us-gaap_StatementClassOfStockAxis=us-gaap_CommonClassAMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
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Subsequent Events - Additional Information (Detail) (USD $)	0 Months Ended
	Apr. 17, 2015	Mar. 31, 2015	Dec. 31, 2014	Apr. 08, 2015	Apr. 10, 2015
Subsequent Event [Line Items]
Number of authorized shares of stock		200,000,000bxmt_StockSharesAuthorized
Number of authorized shares of stock		100,000,000us-gaap_CommonStockSharesAuthorized	100,000,000us-gaap_CommonStockSharesAuthorized
Class A Common Stock [Member]
Subsequent Event [Line Items]
Number of authorized shares of stock		100,000,000
Subsequent Events [Member]
Subsequent Event [Line Items]
Number of authorized shares of stock				300,000,000
Subsequent Events [Member] \| Wells Fargo Facility [Member]
Subsequent Event [Line Items]
Secured financing					$ 4,000,000,000
Subsequent Events [Member] \| Loan Purchase Commitments [Member] \| Commercial Portfolio Segment [Member] \| General Electrical Capital Corporation [Member]
Subsequent Event [Line Items]
Portfolio of loans acquired					4,600,000,000
Purchase price of Loan Portfolio					4,400,000,000
Unfunded commitments assumed					223,900,000
Subsequent Events [Member] \| Class A Common Stock [Member]
Subsequent Event [Line Items]
Number of authorized shares of stock				200,000,000
Subsequent Events [Member] \| Class A Common Stock [Member] \| IPO [Member]
Subsequent Event [Line Items]
Shares issued during period	23,000,000
Net proceeds from underwritten public offering	681,800,000
Underwriters option to purchase additional shares period	30 days
Subsequent Events [Member] \| Class A Common Stock [Member] \| IPO [Member] \| Blackstone Capital Markets [Member]
Subsequent Event [Line Items]
Compensation made to co-manager for stock offering	$ 750,000
Subsequent Events [Member] \| Class A Common Stock [Member] \| IPO [Member] \| Manager [Member] \| Affiliated Entities [Member]
Subsequent Event [Line Items]
Shares purchased	1,229,508
Shares purchased by employees, price per share	$ 30.50
Subsequent Events [Member] \| Class A Common Stock [Member] \| IPO [Member] \| Manager [Member] \| Employees of Affiliates of Manager Including Executive Officers and Directors [Member]
Subsequent Event [Line Items]
Shares purchased	655,737
Shares purchased by employees, price per share	$ 30.50
Subsequent Events [Member] \| Class A Common Stock [Member] \| Over-Allotment Option [Member]
Subsequent Event [Line Items]
Shares issued during period	3,000,000