Transactions with Related Parties - Additional Information (Detail) - USD ($)					1 Months Ended	3 Months Ended			6 Months Ended
	May. 15, 2015	May. 08, 2015	Oct. 23, 2014	Oct. 03, 2013	Apr. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Apr. 10, 2015	Dec. 31, 2014	May. 31, 2013
Related Party Transaction [Line Items]
Issuance of restricted class A common stock									179,799
Unrealized gain on investments at fair value						$ 4,714,000		$ 7,163,000	$ 22,190,000	$ 5,824,000
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			$ 583,000
Common stock, shares issued						93,229,986			93,229,986			58,269,889	25,875,000
Common stock, par value						$ 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			$ 4,700,000,000
IPO [Member] | Blackstone Capital Markets [Member] | Class A Common Stock [Member]
Related Party Transaction [Line Items]
Compensation made to co-manager for stock offering					$ 761,000
Loan Purchase Commitments [Member] | Commercial Portfolio Segment [Member] | General Electrical Capital Corporation [Member]
Related Party Transaction [Line Items]
Portfolio of loans acquired						4,900,000,000			4,900,000,000		$ 4,600,000,000
Purchase price of Loan Portfolio						4,700,000,000			$ 4,700,000,000		$ 23,000,000,000
Restricted Class A Common Stock [Member]
Related Party Transaction [Line Items]
Issuance of restricted class A common stock									190,674
Vesting period of restricted class A common stock									3 years
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 fees payable						8,100,000			$ 8,100,000
Management fees paid to Manager									12,900,000
Preferred distributions payable to affiliate of our Manager									139,000
Manager [Member] | CT Legacy Partners [Member]
Related Party Transaction [Line Items]
Amount of investment in Three-Pack JV												$ 18,500,000
Unrealized gain on investments at fair value									22,100,000
Net sale proceeds assumed									40,600,000
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
Vesting period of restricted class A common stock				3 years
Non-cash expense									3,300,000	$ 1,700,000
BXMT Advisors Limited Liability Company and Affiliates [Member]
Related Party Transaction [Line Items]
Payments of preferred distributions to affiliate of our Manager									841,000
Preferred distributions payable to affiliate of our Manager						$ 83,333			83,333
Modification fee							$ 354,000
Affiliates of Manager [Member] | Third-Party Service Provider [Member]
Related Party Transaction [Line Items]
Administrative services expenses incurred									$ 129,000