Transactions with Related Parties - Additional Information (Detail)	1 Months Ended					3 Months Ended		12 Months Ended
	Aug. 31, 2015 USD ($)	Jun. 30, 2015 USD ($) shares	May 31, 2015 shares	Apr. 30, 2015 USD ($) $ / shares shares	May 31, 2013 shares	Dec. 31, 2015 USD ($) shares	Dec. 31, 2014 USD ($) shares	Dec. 31, 2016 USD ($) SecurityLoan $ / shares shares	Dec. 31, 2015 USD ($) SecurityLoan shares	Dec. 31, 2014 USD ($) shares	Apr. 10, 2015 USD ($)	Jun. 20, 2014 USD ($)
Related Party Transaction [Line Items]
Common stock value						$ 937,000		$ 945,000	$ 937,000
Non-cash expenses								$ 21,899,000	$ 20,717,000	$ 9,716,000
Number of Loans | SecurityLoan								107	125
Loan face amount						9,108,361,000		$ 8,727,218,000	$ 9,108,361,000
Common stock, shares issued | shares					25,875,000			1,070	34,780,622	28,275,006
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
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
CT Legacy Partners [Member]
Related Party Transaction [Line Items]
Total incentive compensation payments								$ 1,094,000	$ 2,177,000	$ 1,374,000
Principal balance of loans												8,000,000
Carrying value of loans												$ 0
Net proceeds to investments from collateral property						$ 0
Restricted Class A Common Stock [Member]
Related Party Transaction [Line Items]
Shares held | shares						1,114,908	919,719	1,309,995	1,114,908	919,719
Restricted shares, vesting period								3 years
Public offing price per share | $ / shares								$ 30.07
Class A Common Stock [Member]
Related Party Transaction [Line Items]
Common stock, shares issued | shares		11,500,000	280,025	23,000,000				0	34,780,025
Manager [Member]
Related Party Transaction [Line Items]
Management Agreement expiration date								Dec. 19, 2017
Management Agreement renewal term, description								The current term of which expires on December 19, 2017, and will be automatically renewed for a one-year term each anniversary thereafter unless earlier terminated.
Management Agreement renewal term, period								1 year
Accrued management and incentive fees payable						$ 14,400,000		$ 12,800,000	$ 14,400,000
Management fees paid to Manager								57,500,000	34,800,000	$ 15,700,000
Expenses reimbursed to Manager								$ 462,000	311,000	115,000
Number of Loans | SecurityLoan								3
Affiliate aggregate fees from loan borrower								$ 346,000
Expenses incurred on insurance agency transaction of affiliate with loan borrower								0
Income received on insurance agency transaction of affiliate with loan borrower								0
Total incentive compensation payments								18,000,000	10,700,000	1,700,000
Manager [Member] | Senior Loan [Member]
Related Party Transaction [Line Items]
Loan face amount								79,000,000
Manager [Member] | CT Legacy Partners [Member]
Related Party Transaction [Line Items]
Net sale proceeds								41,300,000
Net sales proceeds, amount received								3,600,000
Manager [Member] | Incentive Awards Plan [Member] | CT Legacy Partners [Member]
Related Party Transaction [Line Items]
Total incentive compensation payments								2,800,000
Incentive compensation amount paid								$ 244,000	2,500,000
Manager [Member] | Class A Common Stock [Member] | April 2015 Stock Offering [Member]
Related Party Transaction [Line Items]
Common stock, shares issued | shares				1,885,245
Public offing price per share | $ / shares				$ 30.50
Manager [Member] | Class A Common Stock [Member] | Restricted Class A Common Stock [Member]
Related Party Transaction [Line Items]
Shares held | shares								775,961
Common stock value								$ 22,500,000
Non-cash expenses								$ 9,600,000	6,700,000	3,800,000
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
BXMT Advisors Limited Liability Company and Affiliates [Member]
Related Party Transaction [Line Items]
Preferred distributions payable to affiliate of our Manager						83,333			83,333
Payments of preferred distributions to affiliate of our Manager								$ 616,000	1,300,000	2,100,000
Affiliate of our Manager [Member]
Related Party Transaction [Line Items]
Accrued special servicing fee payable						$ 1,100,000			1,100,000
Special servicing fees payable							$ 522,000			522,000
Special servicing fees										139,000
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
Affiliates of Manager [Member] | Third-Party Service Provider [Member]
Related Party Transaction [Line Items]
Administrative services expenses incurred								$ 385,000	$ 372,000	$ 202,000
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