Segment Reporting - Summary of Consolidated Statement of Financial Condition for Each Segment (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Jun. 30, 2012	Dec. 31, 2011
Assets:
Cash and cash equivalents	$ 59,746		$ 15,423	$ 34,604	$ 34,818
Restricted cash	21,972		14,246
Loans receivable, net	830,101		141,500
Loans held-for-sale, net	3,800
Loans receivable, at fair value	117,549
Equity investments in unconsolidated subsidiaries	23,240		13,306
Accrued interest receivable, prepaid expenses, and other assets	30,697
Total assets	1,087,105		322,343
Liabilities:
Accounts payable, accrued expenses and other liabilities	36,879
Secured notes	8,848		8,497
Repurchase obligations	165,239	20,200
Securitized debt obligations	74,472		139,184
Total liabilities	285,438		168,890
Equity:
Total Blackstone Mortgage Trust, Inc. stockholders' equity	712,689		73,444
Non-controlling interests	88,978		80,009
Total equity	801,667		153,453	22,240	(128,939)
Total liabilities and equity	1,087,105		322,343
Loan Origination [Member]
Assets:
Loans receivable, net	753,101
Legacy Portfolio [Member]
Assets:
Loans receivable, net	77,000
Operating Segments [Member] | Loan Origination [Member]
Assets:
Cash and cash equivalents	59,746
Restricted cash
Loans receivable, net	753,101
Loans held-for-sale, net
Equity investments in unconsolidated subsidiaries
Accrued interest receivable, prepaid expenses, and other assets	5,912
Total assets	818,759
Liabilities:
Accounts payable, accrued expenses and other liabilities	4,827
Secured notes
Repurchase obligations	165,239
Securitized debt obligations
Total liabilities	170,066
Equity:
Total Blackstone Mortgage Trust, Inc. stockholders' equity	648,693
Non-controlling interests
Total equity	648,693
Total liabilities and equity	818,759
Operating Segments [Member] | Legacy Portfolio [Member]
Assets:
Cash and cash equivalents
Restricted cash	21,972
Loans receivable, net	77,000
Loans held-for-sale, net	3,800
Loans receivable, at fair value	117,549
Equity investments in unconsolidated subsidiaries	23,240
Accrued interest receivable, prepaid expenses, and other assets	24,785
Total assets	268,346
Liabilities:
Accounts payable, accrued expenses and other liabilities	32,052
Secured notes	8,848
Repurchase obligations
Securitized debt obligations	74,472
Total liabilities	115,372
Equity:
Total Blackstone Mortgage Trust, Inc. stockholders' equity	63,996
Non-controlling interests	88,978
Total equity	152,974
Total liabilities and equity	$ 268,346