Segment Reporting - Consolidated Balance Sheet for Each Segment (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2013	Dec. 31, 2012
Assets
Cash and cash equivalents	$ 70,834	$ 52,342	$ 15,361	$ 15,423
Restricted cash	10,677	10,096
Loans receivable, net	2,710,159	2,047,223
Equity investments in unconsolidated subsidiaries	23,394	22,480
Accrued interest receivable, prepaid expenses, and other assets	79,323	80,639
Total assets	2,894,387	2,212,780
Liabilities and equity
Accounts payable, accrued expenses, and other liabilities	66,711	56,972
Repurchase obligations	1,529,788	1,109,353
Convertible notes, net	160,094	159,524
Securitized debt obligations	39,394	40,181
Participations sold	90,000	90,000
Total liabilities	1,885,987	1,456,030
Equity
Total Blackstone Mortgage Trust, Inc. stockholders' equity	970,083	717,909
Non-controlling interests	38,317	38,841
Total equity	1,008,400	756,750	162,445	153,453
Total liabilities and equity	2,894,387	2,212,780
Loan Origination [Member]
Assets
Cash and cash equivalents	70,834	52,342
Restricted cash
Loans receivable, net	2,663,159	2,000,223
Equity investments in unconsolidated subsidiaries
Accrued interest receivable, prepaid expenses, and other assets	22,163	21,020
Total assets	2,756,156	2,073,585
Liabilities and equity
Accounts payable, accrued expenses, and other liabilities	29,922	21,104
Repurchase obligations	1,529,788	1,109,353
Convertible notes, net	160,094	159,524
Securitized debt obligations
Participations sold	90,000	90,000
Total liabilities	1,809,804	1,379,981
Equity
Total Blackstone Mortgage Trust, Inc. stockholders' equity	946,352	693,604
Non-controlling interests
Total equity	946,352	693,604
Total liabilities and equity	2,756,156	2,073,585
CT Legacy Portfolio [Member]
Assets
Cash and cash equivalents
Restricted cash	10,677	10,096
Loans receivable, net	47,000	47,000
Equity investments in unconsolidated subsidiaries	23,394	22,480
Accrued interest receivable, prepaid expenses, and other assets	57,160	59,619
Total assets	138,231	139,195
Liabilities and equity
Accounts payable, accrued expenses, and other liabilities	36,789	35,868
Repurchase obligations
Convertible notes, net
Securitized debt obligations	39,394	40,181
Participations sold
Total liabilities	76,183	76,049
Equity
Total Blackstone Mortgage Trust, Inc. stockholders' equity	23,731	24,305
Non-controlling interests	38,317	38,841
Total equity	62,048	63,146
Total liabilities and equity	$ 138,231	$ 139,195