Segment Reporting - Consolidated Statement of Operations for Each Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended												12 Months Ended
	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, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Income from loans and other investments
Interest and related income													$ 53,164	$ 34,939	$ 117,161
Less: Interest and related expenses													18,017	38,138	96,974
Income from loans and other investments, net													35,147	(3,199)	20,187
Other expenses
Management fees													5,937
General and administrative expenses													11,505	10,369	8,982
Total other expenses													17,442	10,369	8,982
Valuation allowance on loans held-for-sale													1,259		(1,456)
Unrealized gain on investments at fair value													7,417
Gain on extinguishment of debt													38		271,031
Income before income taxes													26,419	282,387	254,634
Income tax provision													995	174	1,425
Net income	9,728	10,526	6,768	(1,597)	122,931	12,900	3,351	140,622	(16,078)	8,256	6,224	253,917	25,424	279,804	252,319
Net income attributable to non-controlling interests													(10,392)	(98,780)	5,823
Net income (loss) attributable to Blackstone Mortgage Trust, Inc.	7,079	8,320	2,748	(3,115)	105,189	6,999	2,283	66,553	(8,320)	13,722	(1,845)	254,585	15,032	181,024	258,142
Loan Origination [Member]
Income from loans and other investments
Interest and related income													41,621
Less: Interest and related expenses													13,053
Income from loans and other investments, net													28,568
Other expenses
Management fees													5,937
General and administrative expenses													5,149
Total other expenses													11,086
Valuation allowance on loans held-for-sale
Unrealized gain on investments at fair value
Gain on extinguishment of debt
Income before income taxes													17,482
Income tax provision													31
Net income													17,451
Net income attributable to non-controlling interests													(193)
Net income (loss) attributable to Blackstone Mortgage Trust, Inc.													17,258
CT Legacy Portfolio [Member]
Income from loans and other investments
Interest and related income													11,543
Less: Interest and related expenses													4,964
Income from loans and other investments, net													6,579
Other expenses
Management fees
General and administrative expenses													6,356
Total other expenses													6,356
Valuation allowance on loans held-for-sale													1,259
Unrealized gain on investments at fair value													7,417
Gain on extinguishment of debt													38
Income before income taxes													8,937
Income tax provision													964
Net income													7,973
Net income attributable to non-controlling interests													(10,199)
Net income (loss) attributable to Blackstone Mortgage Trust, Inc.													$ (2,226)