Transactions with Related Parties - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Related Party Transaction [Line Items]
Common stock value	$ 939,000		$ 939,000		$ 937,000
CT Legacy Partners [Member]
Related Party Transaction [Line Items]
Total incentive compensation payments	354,000	$ 89,000	$ 1,112,000	$ 2,151,000
Restricted Class A Common Stock [Member]
Related Party Transaction [Line Items]
Restricted shares, vesting period			3 years
Manager [Member]
Related Party Transaction [Line Items]
Management Agreement expiration date			Dec. 19, 2016
Management Agreement renewal term, description			The current 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
Accrued management and incentive fees payable	13,700,000		$ 13,700,000		14,400,000
Management fees paid to Manager	15,800,000	8,100,000	43,800,000	21,000,000
Expenses reimbursed to Manager	82,000	0	462,000	139,000
Insurance agency fee from loan borrower	196,000
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	$ 4,200,000	4,500,000	14,800,000	5,700,000
Manager [Member] | CT Legacy Partners [Member]
Related Party Transaction [Line Items]
Net sale proceeds			40,100,000
Net sales proceeds, amount received			2,400,000
Manager [Member] | Incentive Awards Plan [Member] | CT Legacy Partners [Member]
Related Party Transaction [Line Items]
Total incentive compensation payments					2,700,000
Incentive compensation amount paid			$ 162,000		2,500,000
Manager [Member] | Class A Common Stock [Member] | Restricted Class A Common Stock [Member]
Related Party Transaction [Line Items]
Issuance of restricted class A common stock			0
Shares held	391,871		391,871
Common stock value	$ 10,900,000		$ 10,900,000
Non-cash expense	2,500,000	1,600,000	$ 7,100,000	4,900,000
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
Affiliates of Manager [Member] | Third-Party Service Provider [Member]
Related Party Transaction [Line Items]
Administrative services expenses incurred	112,000	178,000	$ 282,000	307,000
BXMT Advisors Limited Liability Company and Affiliates [Member]
Related Party Transaction [Line Items]
Preferred distributions payable to affiliate of our Manager	31,000		31,000		$ 83,000
Payments of preferred distributions to affiliate of our Manager	$ 146,000	$ 216,000	$ 491,000	$ 1,100,000