Segment Reporting - Consolidated Balance Sheet for Each Segment (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2013	Dec. 31, 2012
Assets
Cash and cash equivalents	$ 120,456	$ 52,342	$ 59,747	$ 15,423
Restricted cash	11,392	10,096
Loans receivable, net	3,488,179	2,047,223
Equity investments in unconsolidated subsidiaries	14,038	22,480
Accrued interest receivable, prepaid expenses, and other assets	120,704	80,639
Total assets	3,754,769	2,212,780
Liabilities and equity
Accounts payable, accrued expenses, and other liabilities	71,345	97,153
Repurchase obligations	1,779,650	1,109,353
Convertible notes, net	160,671	159,524
Participations sold	461,078	90,000
Total liabilities	2,472,744	1,456,030
Equity
Total Blackstone Mortgage Trust, Inc. stockholders' equity	1,239,308	717,909
Non-controlling interests	42,717	38,841
Total equity	1,282,025	756,750	801,667	153,453
Total liabilities and equity	3,754,769	2,212,780
Loan Origination [Member]
Assets
Cash and cash equivalents	120,456	52,342
Loans receivable, net	3,488,179	2,000,223
Accrued interest receivable, prepaid expenses, and other assets	29,803	21,020
Total assets	3,638,438	2,073,585
Liabilities and equity
Accounts payable, accrued expenses, and other liabilities	34,036	21,104
Repurchase obligations	1,779,650	1,109,353
Convertible notes, net	160,671	159,524
Participations sold	461,078	90,000
Total liabilities	2,435,435	1,379,981
Equity
Total Blackstone Mortgage Trust, Inc. stockholders' equity	1,203,003	693,604
Total equity	1,203,003	693,604
Total liabilities and equity	3,638,438	2,073,585
CT Legacy Portfolio [Member]
Assets
Restricted cash	11,392	10,096
Loans receivable, net		47,000
Equity investments in unconsolidated subsidiaries	14,038	22,480
Accrued interest receivable, prepaid expenses, and other assets	90,901	59,619
Total assets	116,331	139,195
Liabilities and equity
Accounts payable, accrued expenses, and other liabilities	37,309	76,049
Total liabilities	37,309	76,049
Equity
Total Blackstone Mortgage Trust, Inc. stockholders' equity	36,305	24,305
Non-controlling interests	42,717	38,841
Total equity	79,022	63,146
Total liabilities and equity	$ 116,331	$ 139,195