Segment Reporting - Consolidated Statement of Operations for Each Segment (Detail) (USD $)	3 Months Ended												12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income from loans and other investments
Interest and related income													$ 184,766,000	$ 53,164,000	$ 34,939,000
Less: Interest and related expenses													69,143,000	18,017,000	38,138,000
Income from loans and other investments, net													115,623,000	35,147,000	(3,199,000)
Other expenses
Management and incentive fees													19,491,000	5,937,000
General and administrative expenses													27,799,000	11,505,000	10,369,000
Total other expenses													47,290,000	17,442,000	10,369,000
Valuation allowance on loans held-for-sale													1,300,000	1,259,000
Gain on investments at fair value													13,258,000	7,417,000	51,904,000
Loss on deconsolidation of subsidiaries													(8,615,000)		200,283,000
Gain on extinguishment of debt														38,000
Income from equity investments in unconsolidated subsidiaries													28,036,000		1,781,000
Income before income taxes													101,012,000	26,419,000	282,387,000
Income tax provision													518,000	995,000	174,000
Net income	25,338,000	23,601,000	38,439,000	13,116,000	9,728,000	10,526,000	6,768,000	(1,597,000)	122,931,000	12,900,000	3,351,000	140,622,000	100,494,000	25,424,000	279,804,000
Net income attributable to non-controlling interests													(10,449,000)	(10,392,000)	(98,780,000)
Net income (loss) attributable to Blackstone Mortgage Trust, Inc.	21,490,000	22,024,000	33,466,000	13,065,000	7,079,000	8,320,000	2,748,000	(3,115,000)	105,189,000	6,999,000	2,283,000	66,553,000	90,045,000	15,032,000	181,024,000
Loan Origination [Member]
Income from loans and other investments
Interest and related income													180,654,000	41,621,000
Less: Interest and related expenses													68,098,000	13,053,000
Income from loans and other investments, net													112,556,000	28,568,000
Other expenses
Management and incentive fees													19,491,000	5,937,000
General and administrative expenses													12,665,000	5,149,000
Total other expenses													32,156,000	11,086,000
Income before income taxes													80,400,000	17,482,000
Income tax provision													194,000	31,000
Net income													80,206,000	17,451,000
Net income attributable to non-controlling interests														(193,000)
Net income (loss) attributable to Blackstone Mortgage Trust, Inc.													80,206,000	17,258,000
CT Legacy Portfolio [Member]
Income from loans and other investments
Interest and related income													4,112,000	11,543,000
Less: Interest and related expenses													1,045,000	4,964,000
Income from loans and other investments, net													3,067,000	6,579,000
Other expenses
General and administrative expenses													15,134,000	6,356,000
Total other expenses													15,134,000	6,356,000
Valuation allowance on loans held-for-sale														1,259,000
Gain on investments at fair value													13,258,000	7,417,000
Loss on deconsolidation of subsidiaries													(8,615,000)
Gain on extinguishment of debt														38,000
Income from equity investments in unconsolidated subsidiaries													28,036,000
Income before income taxes													20,612,000	8,937,000
Income tax provision													324,000	964,000
Net income													20,288,000	7,973,000
Net income attributable to non-controlling interests													(10,449,000)	(10,199,000)
Net income (loss) attributable to Blackstone Mortgage Trust, Inc.													$ 9,839,000	$ (2,226,000)