Transactions with Related Parties - Additional Information (Detail) - USD ($)					1 Months Ended				3 Months Ended		9 Months Ended
	Sep. 30, 2015	May. 08, 2015	Oct. 23, 2014	Oct. 03, 2013	Aug. 31, 2015	May. 31, 2015	Apr. 30, 2015	Mar. 31, 2015	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Jun. 30, 2015	Apr. 10, 2015	Dec. 31, 2014	May. 31, 2013
Related Party Transaction [Line Items]
Amount of loan originated						$ 590,000,000		$ 320,000,000
Class A Common Stock [Member]
Related Party Transaction [Line Items]
Fair value of restricted class A common stock	$ 932,000								$ 932,000		$ 932,000				$ 583,000
Common stock, shares issued	93,212,863								93,212,863		93,212,863				58,269,889	25,875,000
Common stock, par value	$ 0.01								$ 0.01		$ 0.01				$ 0.01
General Electrical Capital Corporation [Member]
Related Party Transaction [Line Items]
Purchase price of Loan Portfolio													$ 4,700,000,000
Blackstone Capital Markets [Member] | IPO [Member] | Class A Common Stock [Member]
Related Party Transaction [Line Items]
Compensation made to co-manager for stock offering							$ 761,000
Restricted Class A Common Stock [Member]
Related Party Transaction [Line Items]
Restricted shares, vesting period											3 years
Loan Purchase Commitments [Member] | Commercial Portfolio Segment [Member] | General Electrical Capital Corporation [Member]
Related Party Transaction [Line Items]
Portfolio of loans acquired														$ 4,600,000,000
Purchase price of Loan Portfolio														$ 23,000,000,000
Manager [Member]
Related Party Transaction [Line Items]
Management Agreement expiration date											Dec. 19, 2016
Management Agreement renewal term, description											The initial term of which expires on December 19, 2016 and will be automatically renewed for a one-year term each anniversary thereafter unless earlier terminated.
Accrued management and incentive fees payable	$ 13,800,000								$ 13,800,000		$ 13,800,000
Management fees paid to Manager									8,100,000	$ 4,400,000	21,000,000	$ 10,300,000
Expenses reimbursed to Manager									0	25,000	139,000	115,000
Manager [Member] | CT Legacy Partners [Member]
Related Party Transaction [Line Items]
Amount of investment in Three-Pack JV															$ 18,500,000
Recognized gain on investments											22,100,000
Net sale proceeds assumed											40,500,000
Expected net sales proceeds, amount received	37,700,000
Manager [Member] | CT Legacy Partners [Member] | Incentive Awards Plan [Member]
Related Party Transaction [Line Items]
Aggregate incentive compensation payments		$ 2,700,000
Incentive compensation amount paid		$ 2,500,000
Manager [Member] | IPO [Member] | Class A Common Stock [Member]
Related Party Transaction [Line Items]
Common stock, shares issued							1,885,245
Common stock, par value							$ 30.50
Manager [Member] | Restricted Class A Common Stock [Member] | Class A Common Stock [Member]
Related Party Transaction [Line Items]
Issuance of restricted class A common stock			337,941	339,431
Fair value of restricted class A common stock			$ 9,400,000	$ 8,500,000
Restricted shares, vesting period				3 years
Non-cash expense									1,600,000	766,000	4,900,000	2,400,000
BXMT Advisors Limited Liability Company and Affiliates [Member]
Related Party Transaction [Line Items]
Payments of preferred distributions to affiliate of our Manager									216,000	533,000	1,100,000	1,700,000
Preferred distributions payable to affiliate of our Manager	$ 83,333								83,333		83,333				$ 151,000
Modification fee								$ 354,000
Independent Members of Board of Directors [Member] | Loan Purchase Commitments [Member] | Commercial Portfolio Segment [Member] | General Electrical Capital Corporation [Member]
Related Party Transaction [Line Items]
Payments for special committee services					$ 53,000
Affiliates of Manager [Member] | Third-Party Service Provider [Member]
Related Party Transaction [Line Items]
Administrative services expenses incurred									$ 178,000	$ 79,000	$ 307,000	$ 141,000