Segment Reporting - Consolidated Statement of Operations for Each Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Income from loans and other investments
Interest and related income	$ 33,656	$ 1,456
Less: Interest and related expenses	12,074	777
Income from loans and other investments, net	21,582	679
Other expenses
Management fees	3,397	63
General and administrative expenses	3,199	1,975
Total other expenses	6,596	2,038
Loss on investments at fair value	(1,339)
Valuation allowance on loans held-for-sale		(200)
Income (loss) before income taxes	13,647	(1,559)
Income tax provision	531	38
Net income (loss)	13,116	(1,597)
Net income attributable to non-controlling interests	(51)	(1,518)
Net income (loss) attributable to Blackstone Mortgage Trust, Inc.	13,065	(3,115)
Loan Origination [Member]
Income from loans and other investments
Interest and related income	32,035
Less: Interest and related expenses	11,626
Income from loans and other investments, net	20,409
Other expenses
Management fees	3,397
General and administrative expenses	2,846
Total other expenses	6,243
Loss on investments at fair value
Valuation allowance on loans held-for-sale
Income (loss) before income taxes	14,166
Income tax provision	131
Net income (loss)	14,035
Net income attributable to non-controlling interests
Net income (loss) attributable to Blackstone Mortgage Trust, Inc.	14,035
CT Legacy Portfolio [Member]
Income from loans and other investments
Interest and related income	1,621	1,456
Less: Interest and related expenses	448	777
Income from loans and other investments, net	1,173	679
Other expenses
Management fees		63
General and administrative expenses	353	1,975
Total other expenses	353	2,038
Loss on investments at fair value	(1,339)
Valuation allowance on loans held-for-sale		(200)
Income (loss) before income taxes	(519)	(1,559)
Income tax provision	400	38
Net income (loss)	(919)	(1,597)
Net income attributable to non-controlling interests	(51)	(1,518)
Net income (loss) attributable to Blackstone Mortgage Trust, Inc.	$ (970)	$ (3,115)