Segment Reporting - Consolidated Balance Sheet for Each Segment (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2013	Dec. 31, 2012
Assets
Cash and cash equivalents	$ 63,343	$ 52,342	$ 10,283	$ 15,423
Restricted cash	10,855	10,096
Loans receivable, net	3,906,226	2,047,223
Equity investments in unconsolidated subsidiaries	14,990	22,480
Accrued interest receivable, prepaid expenses, and other assets	103,059	80,639
Total assets	4,098,473	2,212,780
Liabilities and equity
Accounts payable, accrued expenses, and other liabilities	83,011	97,153
Revolving repurchase facilities	1,669,406	863,622
Asset-specific repurchase agreements	226,961	245,731
Loan participations sold	447,977	90,000
Convertible notes, net	161,259	159,524
Total liabilities	2,588,614	1,456,030
Equity
Total Blackstone Mortgage Trust, Inc. stockholders' equity	1,477,739	717,909
Non-controlling interests	32,120	38,841
Total equity	1,509,859	756,750	803,542	153,453
Total liabilities and equity	4,098,473	2,212,780
Loan Origination [Member]
Assets
Cash and cash equivalents	63,343	52,342
Loans receivable, net	3,906,226	2,000,223
Accrued interest receivable, prepaid expenses, and other assets	29,727	21,020
Total assets	3,999,296	2,073,585
Liabilities and equity
Accounts payable, accrued expenses, and other liabilities	45,309	21,104
Revolving repurchase facilities	1,669,406	863,622
Asset-specific repurchase agreements	226,961	245,731
Loan participations sold	447,977	90,000
Convertible notes, net	161,259	159,524
Total liabilities	2,550,912	1,379,981
Equity
Total Blackstone Mortgage Trust, Inc. stockholders' equity	1,448,384	693,604
Total equity	1,448,384	693,604
Total liabilities and equity	3,999,296	2,073,585
CT Legacy Portfolio [Member]
Assets
Restricted cash	10,855	10,096
Loans receivable, net		47,000
Equity investments in unconsolidated subsidiaries	14,990	22,480
Accrued interest receivable, prepaid expenses, and other assets	73,332	59,619
Total assets	99,177	139,195
Liabilities and equity
Accounts payable, accrued expenses, and other liabilities	37,702	76,049
Total liabilities	37,702	76,049
Equity
Total Blackstone Mortgage Trust, Inc. stockholders' equity	29,355	24,305
Non-controlling interests	32,120	38,841
Total equity	61,475	63,146
Total liabilities and equity	$ 99,177	$ 139,195