Segment Reporting - Consolidated Balance Sheet for Each Segment (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Mar. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Assets
Cash and cash equivalents	$ 52,342		$ 15,423	$ 34,818	$ 24,449
Restricted cash	10,096		14,246
Loans receivable, net	2,047,223		141,500
Loans receivable, at fair value	40,665
Equity investments in unconsolidated subsidiaries	22,480		13,306
Accrued interest receivable, prepaid expenses, and other assets	39,974
Total assets	2,212,780		322,343
Liabilities and Equity
Accounts payable, accrued expenses, and other liabilities	56,972
Repurchase obligations	1,109,353	20,200
Convertible notes, net	159,524
Securitized debt obligations	40,181		139,184
Participations sold	90,000
Total liabilities	1,456,030		168,890
Equity
Total Blackstone Mortgage Trust, Inc. stockholders' equity	717,909		73,444
Non-controlling interests	38,841		80,009
Total equity	756,750		153,453	(128,939)	(411,187)
Total liabilities and equity	2,212,780		322,343
Loan Origination [Member]
Assets
Cash and cash equivalents	52,342
Restricted cash
Loans receivable, net	2,000,223
Loans receivable, at fair value
Equity investments in unconsolidated subsidiaries
Accrued interest receivable, prepaid expenses, and other assets	21,020
Total assets	2,073,585
Liabilities and Equity
Accounts payable, accrued expenses, and other liabilities	21,104
Repurchase obligations	1,109,353
Convertible notes, net	159,524
Securitized debt obligations
Participations sold	90,000
Total liabilities	1,379,981
Equity
Total Blackstone Mortgage Trust, Inc. stockholders' equity	693,604
Non-controlling interests
Total equity	693,604
Total liabilities and equity	2,073,585
CT Legacy Portfolio [Member]
Assets
Cash and cash equivalents
Restricted cash	10,096
Loans receivable, net	47,000
Loans receivable, at fair value	40,665
Equity investments in unconsolidated subsidiaries	22,480
Accrued interest receivable, prepaid expenses, and other assets	18,954
Total assets	139,195
Liabilities and Equity
Accounts payable, accrued expenses, and other liabilities	35,868
Repurchase obligations
Convertible notes, net
Securitized debt obligations	40,181
Participations sold
Total liabilities	76,049
Equity
Total Blackstone Mortgage Trust, Inc. stockholders' equity	24,305
Non-controlling interests	38,841
Total equity	63,146
Total liabilities and equity	$ 139,195