Segment Reporting - Summary of Consolidated Statement of Financial Condition for Each Segment (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Dec. 31, 2011
Assets
Cash and cash equivalents	$ 10,283		$ 15,423	$ 38,867	$ 34,818
Restricted cash	76,396		14,246
Loans receivable, net	1,362,891		141,500
Loans receivable, at fair value	66,063
Equity investments in unconsolidated subsidiaries	25,632		13,306
Accrued interest receivable, prepaid expenses, and other assets	36,045
Total assets	1,577,310		322,343
Liabilities
Accounts payable, accrued expenses and other liabilities	47,495
Secured notes	9,030		8,497
Repurchase obligations	643,040	20,200
Securitized debt obligations	74,203		139,184
Total liabilities	773,768		168,890
Equity
Total Blackstone Mortgage Trust, Inc. stockholders' equity	713,327		73,444
Non-controlling interests	90,215		80,009
Total equity	803,542		153,453	37,966	(128,939)
Total liabilities and equity	1,577,310		322,343
Loan Origination [Member]
Assets
Loans receivable, net	1,285,891
CT Legacy Portfolio [Member]
Assets
Loans receivable, net	77,000
Operating Segments [Member] | Loan Origination [Member]
Assets
Cash and cash equivalents	10,283
Restricted cash
Loans receivable, net	1,285,891
Loans receivable, at fair value
Equity investments in unconsolidated subsidiaries
Accrued interest receivable, prepaid expenses, and other assets	8,655
Total assets	1,304,829
Liabilities
Accounts payable, accrued expenses and other liabilities	12,781
Secured notes
Repurchase obligations	643,040
Securitized debt obligations
Total liabilities	655,821
Equity
Total Blackstone Mortgage Trust, Inc. stockholders' equity	649,008
Non-controlling interests
Total equity	649,008
Total liabilities and equity	1,304,829
Operating Segments [Member] | CT Legacy Portfolio [Member]
Assets
Cash and cash equivalents
Restricted cash	76,396
Loans receivable, net	77,000
Loans receivable, at fair value	66,063
Equity investments in unconsolidated subsidiaries	25,632
Accrued interest receivable, prepaid expenses, and other assets	27,390
Total assets	272,481
Liabilities
Accounts payable, accrued expenses and other liabilities	34,714
Secured notes	9,030
Repurchase obligations
Securitized debt obligations	74,203
Total liabilities	117,947
Equity
Total Blackstone Mortgage Trust, Inc. stockholders' equity	64,319
Non-controlling interests	90,215
Total equity	154,534
Total liabilities and equity	$ 272,481