Segment Reporting - Results of Operations for Each Segment (Detail) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Income from loans and other investments:
Interest and related income	$ 6,017,000	$ 6,763,000	$ 7,473,000	$ 21,479,000
Less: Interest and related expenses	1,306,000	5,413,000	2,083,000	28,754,000
Income from loans and other investments, net	4,711,000	1,350,000	5,390,000	(7,275,000)
Other expenses:
General and administrative	3,427,000	1,567,000	5,465,000	2,323,000
Total other expenses	3,427,000	1,567,000	5,465,000	2,323,000
Valuation allowance on loans held-for-sale	2,000,000		1,800,000
Unrealized gain on investments at fair value	4,000,000		4,000,000
Gain on extinguishment of debt	38,000		38,000
Income before income taxes	7,322,000	3,692,000	5,763,000	145,188,000
Income tax provision	554,000		593,000	301,000
Net Income	6,768,000	3,351,000	5,170,000	143,973,000
Net income attributable to non-controlling interests	(4,020,000)	(1,068,000)	(5,537,000)	(75,137,000)
Net (loss) income attributable to Blackstone Mortgage Trust, Inc.	2,748,000	2,283,000	(367,000)	68,836,000
Operating Segments [Member] | Loan Origination [Member]
Income from loans and other investments:
Interest and related income	1,908,000		1,908,000
Less: Interest and related expenses	168,000		168,000
Income from loans and other investments, net	1,740,000		1,740,000
Other expenses:
General and administrative	2,153,000		2,883,000
Total other expenses	2,153,000		2,883,000
Valuation allowance on loans held-for-sale
Unrealized gain on investments at fair value
Gain on extinguishment of debt
Income before income taxes	(413,000)		(1,143,000)
Income tax provision	2,000		40,000
Net Income	(415,000)		(1,183,000)
Net income attributable to non-controlling interests
Net (loss) income attributable to Blackstone Mortgage Trust, Inc.	(415,000)		(1,183,000)
Operating Segments [Member] | Legacy Portfolio [Member]
Income from loans and other investments:
Interest and related income	4,109,000		5,565,000
Less: Interest and related expenses	1,138,000		1,915,000
Income from loans and other investments, net	2,971,000		3,650,000
Other expenses:
General and administrative	1,274,000		2,582,000
Total other expenses	1,274,000		2,582,000
Valuation allowance on loans held-for-sale	2,000,000		1,800,000
Unrealized gain on investments at fair value	4,000,000		4,000,000
Gain on extinguishment of debt	38,000		38,000
Income before income taxes	7,735,000		6,906,000
Income tax provision	552,000		553,000
Net Income	7,183,000		6,353,000
Net income attributable to non-controlling interests	(4,020,000)		(5,537,000)
Net (loss) income attributable to Blackstone Mortgage Trust, Inc.	$ 3,163,000		$ 816,000