Transactions with Related Parties - Additional Information (Detail) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
CT Legacy Partners [Member]
Related Party Transaction [Line Items]
Total incentive compensation payments	$ 129,000	$ 1,030,000
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 value	$ 939,000		$ 937,000
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		14,400,000
Management fees paid to Manager	14,400,000	6,300,000
Expenses reimbursed to Manager	320,000	139,000
Total incentive compensation payments	4,200,000	1,200,000
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		334,758	337,941	339,431
Common stock value			$ 9,300,000	$ 9,400,000	$ 8,500,000
Non-cash expense	$ 2,100,000	1,400,000
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	$ 90,000	75,000
BXMT Advisors Limited Liability Company and Affiliates [Member]
Related Party Transaction [Line Items]
Preferred distributions payable to affiliate of our Manager	44,000		$ 83,000
Payments of preferred distributions to affiliate of our Manager	$ 224,000	$ 452,000