Segment Reporting - Consolidated Balance Sheet for Each Segment (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Assets
Cash and cash equivalents	$ 51,810,000	$ 52,342,000	$ 15,423,000	$ 34,818,000
Restricted cash	11,591,000	10,096,000
Loans receivable, net	4,428,500,000	2,047,223,000
Equity investments in unconsolidated subsidiaries	10,604,000	22,480,000
Accrued interest receivable, prepaid expenses, and other assets	86,016,000	80,639,000
Total assets	4,588,521,000	2,212,780,000
Liabilities and Equity
Accounts payable, accrued expenses, and other liabilities	61,013,000	97,153,000
Revolving repurchase facilities	2,040,783,000	863,622,000
Asset-specific repurchase agreements	324,553,000	245,731,000
Loan participations sold	499,433,000	90,000,000
Convertible notes, net	161,853,000	159,524,000
Total liabilities	3,087,635,000	1,456,030,000
Equity
Total Blackstone Mortgage Trust, Inc. stockholders' equity	1,465,371,000	717,909,000
Non-controlling interests	35,515,000	38,841,000
Total equity	1,500,886,000	756,750,000	153,453,000	(128,939,000)
Total liabilities and equity	4,588,521,000	2,212,780,000
Loan Origination [Member]
Assets
Cash and cash equivalents	51,810,000	52,342,000
Loans receivable, net	4,428,500,000	2,000,223,000
Accrued interest receivable, prepaid expenses, and other assets	36,531,000	21,020,000
Total assets	4,516,841,000	2,073,585,000
Liabilities and Equity
Accounts payable, accrued expenses, and other liabilities	47,328,000	21,104,000
Revolving repurchase facilities	2,040,783,000	863,622,000
Asset-specific repurchase agreements	324,553,000	245,731,000
Loan participations sold	499,433,000	90,000,000
Convertible notes, net	161,853,000	159,524,000
Total liabilities	3,073,950,000	1,379,981,000
Equity
Total Blackstone Mortgage Trust, Inc. stockholders' equity	1,442,891,000	693,604,000
Total equity	1,442,891,000	693,604,000
Total liabilities and equity	4,516,841,000	2,073,585,000
CT Legacy Portfolio [Member]
Assets
Restricted cash	11,591,000	10,096,000
Loans receivable, net		47,000,000
Equity investments in unconsolidated subsidiaries	10,604,000	22,480,000
Accrued interest receivable, prepaid expenses, and other assets	49,485,000	59,619,000
Total assets	71,680,000	139,195,000
Liabilities and Equity
Accounts payable, accrued expenses, and other liabilities	13,685,000	76,049,000
Total liabilities	13,685,000	76,049,000
Equity
Total Blackstone Mortgage Trust, Inc. stockholders' equity	22,480,000	24,305,000
Non-controlling interests	35,515,000	38,841,000
Total equity	57,995,000	63,146,000
Total liabilities and equity	$ 71,680,000	$ 139,195,000