Transactions with Related Parties - Additional Information (Detail) (USD $)	3 Months Ended		0 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Oct. 23, 2014	Oct. 03, 2013	Dec. 31, 2014
Related Party Transaction [Line Items]
Management Agreement expiration date	Dec. 19, 2015
Management Agreement renewal term, description	The initial term of which expires on December 19, 2015 and will be automatically renewed for a one-year term each anniversary thereafter unless earlier terminated.
Issuance of restricted class A common stock	181,049
Fair value of restricted class A common stock	$ 585,000				$ 583,000
Amount of loan originated	320,000,000
Unrealized gain on investments at fair value	17,476,000	(1,339,000)
Third-Party Service Provider [Member] | Equity Capital Markets Data Services [Member]
Related Party Transaction [Line Items]
Service expenses paid to third-party service provider	18,000
Restricted Class A Common Stock [Member]
Related Party Transaction [Line Items]
Issuance of restricted class A common stock	187,674
Vesting period of restricted class A common stock	3 years
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	17,600,000
Net sale proceeds assumed	36,100,000
CT Legacy Partners [Member] | Incentive Awards Plan [Member]
Related Party Transaction [Line Items]
Aggregate incentive compensation payments	2,400,000
Manager [Member]
Related Party Transaction [Line Items]
Accrued management fees payable	6,700,000
Management fees paid to Manager	6,300,000
Preferred distributions payable to affiliate of our Manager	139,000
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	1,400,000	859,000
BXMT Advisors Limited Liability Company and Affiliates [Member]
Related Party Transaction [Line Items]
Payments of preferred distributions to affiliate of our Manager	452,000
Preferred distributions payable to affiliate of our Manager	151,000
Administrative services expenses incurred	20,000
Modification fee	354,000
Affiliate of Manager [Member] | Third-Party Service Provider [Member]
Related Party Transaction [Line Items]
Acquired service provider, date	Aug. 06, 2014
Administrative services expenses incurred	$ 37,000