Segment Reporting - Consolidated Statement of Operations for Each Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Income from loans and other investments
Interest and related income	$ 50,386	$ 18,853	$ 126,507	$ 26,327
Less: Interest and related expenses	19,903	4,407	47,697	6,492
Income from loans and other investments, net	30,483	14,446	78,810	19,835
Other expenses
Management and incentive fees	5,412	2,433	13,219	3,416
General and administrative expenses	3,368	1,615	21,920	6,096
Total other expenses	8,780	4,048	35,139	9,512
Valuation allowance on loans held-for-sale		(600)		1,200
Gain on investments at fair value	1,780	464	7,604	4,464
Gain on extinguishment of debt				38
Income from equity investments in unconsolidated subsidiaries			24,294
Income before income taxes	23,483	10,262	75,569	16,025
Income tax benefit	(118)	(264)	412	329
Net income	23,601	10,526	75,157	15,696
Net income attributable to non-controlling interests	(1,577)	(2,206)	(6,602)	(7,743)
Net income attributable to Blackstone Mortgage Trust, Inc.	22,024	8,320	68,555	7,953
Loan Origination [Member]
Income from loans and other investments
Interest and related income	49,720	15,143	123,127	17,051
Less: Interest and related expenses	19,713	3,822	46,843	3,991
Income from loans and other investments, net	30,007	11,321	76,284	13,060
Other expenses
Management and incentive fees	5,412	2,433	13,219	3,416
General and administrative expenses	2,705	1,026	9,049	2,923
Total other expenses	8,117	3,459	22,268	6,339
Income before income taxes	21,890	7,862	54,016	6,721
Income tax benefit		(21)	131	(19)
Net income	21,890	7,883	53,885	6,740
Net income attributable to non-controlling interests				(193)
Net income attributable to Blackstone Mortgage Trust, Inc.	21,890	7,883	53,885	6,547
CT Legacy Portfolio [Member]
Income from loans and other investments
Interest and related income	666	3,710	3,380	9,276
Less: Interest and related expenses	190	585	854	2,501
Income from loans and other investments, net	476	3,125	2,526	6,775
Other expenses
General and administrative expenses	663	589	12,871	3,173
Total other expenses	663	589	12,871	3,173
Valuation allowance on loans held-for-sale		(600)		1,200
Gain on investments at fair value	1,780	464	7,604	4,464
Gain on extinguishment of debt				38
Income from equity investments in unconsolidated subsidiaries			24,294
Income before income taxes	1,593	2,400	21,553	9,304
Income tax benefit	(118)	(243)	281	348
Net income	1,711	2,643	21,272	8,956
Net income attributable to non-controlling interests	(1,577)	(2,206)	(6,602)	(7,550)
Net income attributable to Blackstone Mortgage Trust, Inc.	$ 134	$ 437	$ 14,670	$ 1,406