Segment Reporting - Consolidated Statement of Operations for Each Segment (Detail) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Income from loans and other investments
Interest and related income	$ 42,466,000	$ 6,017,000	$ 76,122,000	$ 7,473,000
Less: Interest and related expenses	15,720,000	1,306,000	27,794,000	2,083,000
Income from loans and other investments, net	26,746,000	4,711,000	48,328,000	5,390,000
Other expenses
Management fees	4,410,000	920,000	7,807,000	983,000
General and administrative expenses	15,356,000	2,507,000	18,554,000	4,482,000
Total other expenses	19,766,000	3,427,000	26,361,000	5,465,000
Valuation allowance on loans held-for-sale	0	2,000,000	0	1,800,000
Gain on investments at fair value	7,163,000	4,000,000	5,824,000	4,000,000
Gain on extinguishment of debt	0	38,000	0	38,000
Income from equity investments in unconsolidated subsidiaries	24,294,000		24,294,000
(Loss) income before income taxes	38,437,000	7,322,000	52,085,000	5,763,000
Income tax (benefit) provision	(2,000)	554,000	530,000	593,000
Net (loss) income	38,439,000	6,768,000	51,555,000	5,170,000
Net income attributable to non-controlling interests	(4,973,000)	(4,020,000)	(5,024,000)	(5,537,000)
Net (loss) income attributable to Blackstone Mortgage Trust, Inc.	33,466,000	2,748,000	46,531,000	(367,000)
Loan Origination [Member]
Income from loans and other investments
Interest and related income	41,372,000	1,908,000	73,408,000	1,908,000
Less: Interest and related expenses	15,503,000	168,000	27,130,000	168,000
Income from loans and other investments, net	25,869,000	1,740,000	46,278,000	1,740,000
Other expenses
Management fees	4,410,000	920,000	7,807,000	983,000
General and administrative expenses	3,501,000	1,233,000	6,346,000	1,900,000
Total other expenses	7,911,000	2,153,000	14,153,000	2,883,000
(Loss) income before income taxes	17,958,000	(413,000)	32,125,000	(1,143,000)
Income tax (benefit) provision		2,000	131,000	40,000
Net (loss) income	17,958,000	(415,000)	31,994,000	(1,183,000)
Net (loss) income attributable to Blackstone Mortgage Trust, Inc.	17,958,000	(415,000)	31,994,000	(1,183,000)
CT Legacy Portfolio [Member]
Income from loans and other investments
Interest and related income	1,094,000	4,109,000	2,714,000	5,565,000
Less: Interest and related expenses	217,000	1,138,000	664,000	1,915,000
Income from loans and other investments, net	877,000	2,971,000	2,050,000	3,650,000
Other expenses
General and administrative expenses	11,855,000	1,274,000	12,208,000	2,582,000
Total other expenses	11,855,000	1,274,000	12,208,000	2,582,000
Valuation allowance on loans held-for-sale		2,000,000		1,800,000
Gain on investments at fair value	7,163,000	4,000,000	5,824,000	4,000,000
Gain on extinguishment of debt		38,000		38,000
Income from equity investments in unconsolidated subsidiaries	24,294,000		24,294,000
(Loss) income before income taxes	20,479,000	7,735,000	19,960,000	6,906,000
Income tax (benefit) provision	(2,000)	552,000	399,000	553,000
Net (loss) income	20,481,000	7,183,000	19,561,000	6,353,000
Net income attributable to non-controlling interests	(4,973,000)	(4,020,000)	(5,024,000)	(5,537,000)
Net (loss) income attributable to Blackstone Mortgage Trust, Inc.	$ 15,508,000	$ 3,163,000	$ 14,537,000	$ 816,000