Segment Reporting - Results of Operations for Each Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Income from loans and other investments
Interest and related income	$ 18,853	$ 6,944	$ 26,327	$ 28,423
Less: Interest and related expenses	4,407	5,147	6,492	33,902
Income from loans and other investments, net	14,446	1,797	19,835	(5,479)
Other expenses
General and administrative	4,048	3,991	9,512	6,314
Total other expenses	4,048	3,991	9,512	6,314
Valuation allowance on loans held-for-sale	(600)		1,200
Unrealized gain on investments at fair value	464		4,464
Gain on extinguishment of debt			38
Income before income taxes	10,262	13,015	16,025	158,203
Income tax (benefit) provision	(264)	166	329	467
Net income	10,526	12,900	15,696	156,873
Net income attributable to non-controlling interests	(2,206)	(5,901)	(7,743)	(81,038)
Net income attributable to Blackstone Mortgage Trust, Inc.	8,320	6,999	7,953	75,835
Loan Origination [Member]
Income from loans and other investments
Interest and related income	15,143		17,051
Less: Interest and related expenses	3,822		3,991
Income from loans and other investments, net	11,321		13,060
Other expenses
General and administrative	3,459		6,339
Total other expenses	3,459		6,339
Valuation allowance on loans held-for-sale
Unrealized gain on investments at fair value
Gain on extinguishment of debt
Income before income taxes	7,862		6,721
Income tax (benefit) provision	(21)		(19)
Net income	7,883		6,740
Net income attributable to non-controlling interests			(193)
Net income attributable to Blackstone Mortgage Trust, Inc.	7,883		6,547
CT Legacy Portfolio [Member]
Income from loans and other investments
Interest and related income	3,710		9,276
Less: Interest and related expenses	585		2,501
Income from loans and other investments, net	3,125		6,775
Other expenses
General and administrative	589		3,173
Total other expenses	589		3,173
Valuation allowance on loans held-for-sale	(600)		1,200
Unrealized gain on investments at fair value	464		4,464
Gain on extinguishment of debt			38
Income before income taxes	2,400		9,304
Income tax (benefit) provision	(243)		348
Net income	2,643		8,956
Net income attributable to non-controlling interests	(2,206)		(7,550)
Net income attributable to Blackstone Mortgage Trust, Inc.	$ 437		$ 1,406