Transactions with Related Parties - Additional Information (Detail) - USD ($)						1 Months Ended				3 Months Ended		12 Months Ended
	Dec. 31, 2015	May. 08, 2015	Oct. 23, 2013	Oct. 02, 2013	May. 13, 2013	Aug. 31, 2015	May. 31, 2015	Apr. 30, 2015	Mar. 31, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Feb. 09, 2016	Jun. 30, 2015	Apr. 10, 2015	Jun. 20, 2014	Nov. 19, 2013	May. 30, 2013	May. 22, 2013
Related Party Transaction [Line Items]
Joint venture agreement date												May 13, 2013
Joint venture agreement end date												May 30, 2013
Percentage of equity interest purchased					100.00%															100.00%
Charge to non-controlling interest from purchase of equity interests at fair value					$ 193,000
Convertible notes	$ 164,026,000									$ 164,026,000	$ 161,455,000	$ 164,026,000	$ 161,455,000
Amount of loan originated									$ 320,000,000			33,501,000	35,449,000	$ 25,402,000
Purchased loan from third party			$ 176,900,000
Loss on deconsolidation													8,615,000
CT CDO I [Member]
Related Party Transaction [Line Items]
Principal balance of loans																		$ 27,000,000
Carrying value of loans																		27,000,000
Loss on deconsolidation											$ 8,600,000
Net proceeds to investments from collateral property										0
CT Legacy Partners [Member]
Related Party Transaction [Line Items]
Principal balance of loans																		8,000,000
Carrying value of loans																		$ 0
Net proceeds to investments from collateral property										$ 0
Aggregate incentive compensation payments												$ 2,177,000	$ 1,374,000	5,089,000
General Electrical Capital Corporation [Member]
Related Party Transaction [Line Items]
Purchase price of Loan Portfolio																$ 4,700,000,000
General Electrical Capital Corporation [Member] | Loan Purchase Commitments [Member] | Commercial Portfolio Segment [Member]
Related Party Transaction [Line Items]
Portfolio of loans acquired																	$ 4,600,000,000
Purchase price of Loan Portfolio																	$ 23,000,000,000
Restricted Class A Common Stock [Member]
Related Party Transaction [Line Items]
Restricted shares, vesting period												3 years
Class A Common Stock [Member]
Related Party Transaction [Line Items]
Common stock, shares issued	93,702,326									93,702,326	58,269,889	93,702,326	58,269,889
Common stock, par value	$ 0.01									$ 0.01	$ 0.01	$ 0.01	$ 0.01
Common stock value	$ 937,000									$ 937,000	$ 583,000	$ 937,000	$ 583,000
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.
Management Agreement renewal term, period												1 year
Aggregate incentive compensation payments												$ 10,700,000	1,700,000	0
Accrued management and incentive fees payable	14,400,000									14,400,000	6,300,000	14,400,000	6,300,000
Management fees paid to Manager												34,800,000	15,700,000	3,400,000
Expenses reimbursed to Manager												311,000	115,000	0
Manager [Member] | CT Legacy Partners [Member]
Related Party Transaction [Line Items]
Recognized gain on investments												21,600,000
Net sale proceeds assumed												40,100,000
Expected net sales proceeds, amount received	37,700,000
Manager [Member] | Incentive Awards Plan [Member] | CT Legacy Partners [Member]
Related Party Transaction [Line Items]
Aggregate incentive compensation payments		$ 2,700,000
Incentive compensation amount paid		$ 2,500,000
Manager [Member] | Class A Common Stock [Member] | April 2015 Stock Offering [Member]
Related Party Transaction [Line Items]
Common stock, shares issued								1,885,245
Public offing price per share								$ 30.50
Manager [Member] | Class A Common Stock [Member] | Restricted Class A Common Stock [Member]
Related Party Transaction [Line Items]
Common stock value	9,300,000									9,300,000	9,400,000	$ 9,300,000	$ 9,400,000	$ 8,500,000
Issuance of restricted class A common stock												334,758	337,941	339,431
Non-cash expense												$ 6,700,000	$ 3,800,000	$ 767,000
Manager [Member] | Class A Common Stock [Member] | Restricted Class A Common Stock [Member] | Stock Incentive Current Plan [Member]
Related Party Transaction [Line Items]
Restricted shares, vesting period												3 years
Blackstone [Member]
Related Party Transaction [Line Items]
Percentage of equity interest purchased in joint venture					16.70%
Affiliates of Manager [Member]
Related Party Transaction [Line Items]
Percentage of equity interest purchased in joint venture					83.30%
Convertible notes																			$ 188,000
Amount of loan originated				$ 71,000,000			$ 590,000,000
Modification fee			$ 2,300,000
Affiliates of Manager [Member] | Third-Party Service Provider [Member]
Related Party Transaction [Line Items]
Administrative services expenses incurred												$ 372,000	202,000	9,000
Affiliates of Manager [Member] | Class A Common Stock [Member]
Related Party Transaction [Line Items]
Common stock, shares issued																					1,960,784
Common stock, par value																					$ 25.50
Common stock value																					$ 1,000,000
Affiliates of Manager [Member] | Class A Common Stock [Member] | Subsequent Events [Member]
Related Party Transaction [Line Items]
Common stock, shares outstanding percentage															4.70%
Affiliates of Manager [Member] | Class A Common Stock [Member] | April 2015 Stock Offering [Member]
Related Party Transaction [Line Items]
Compensation made to co-manager for stock offering								$ 761,000
Affiliate of our Manager [Member]
Related Party Transaction [Line Items]
Special servicing fees payable											522,000		522,000	847,000
Special servicing fees													139,000	847,000
Accrued special servicing fee payable	1,100,000									1,100,000		1,100,000
Affiliate of our Manager [Member] | CT CDO I [Member]
Related Party Transaction [Line Items]
Special servicing fees payable	0									0	522,000	0	522,000	847,000
Independent Members of Board of Directors [Member] | General Electrical Capital Corporation [Member] | Loan Purchase Commitments [Member] | Commercial Portfolio Segment [Member]
Related Party Transaction [Line Items]
Payments for special committee services						$ 53,000
BXMT Advisors Limited Liability Company and Affiliates [Member]
Related Party Transaction [Line Items]
Modification fee									$ 354,000
Preferred distributions payable to affiliate of our Manager	$ 83,333									$ 83,333	$ 151,000	83,333	151,000
Payments of preferred distributions to affiliate of our Manager												$ 1,300,000	$ 2,100,000	$ 3,900,000