Equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands				1 Months Ended			3 Months Ended		6 Months Ended
	Jul. 15, 2015	Jun. 30, 2015	May. 09, 2014	Jun. 30, 2015	May. 31, 2015	Apr. 30, 2015	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014	Mar. 25, 2014	May. 31, 2013
Class of Stock [Line Items]
Total stock, shares authorized		300,000,000		300,000,000			300,000,000		300,000,000
Preferred stock, shares authorized		100,000,000		100,000,000			100,000,000		100,000,000
Preferred stock issued		0		0			0		0
Preferred stock outstanding		0		0			0		0
Plan adoption date									Mar. 25, 2014
Number of shares sold during period					280,025				34,780,298	18,975,001
Net proceeds from issuance of common stock					$ 7,900				$ 1,029,557	$ 510,845
Dividends paid									$ 60,695	$ 32,129
Dividends declared per common stock		$ 0.52					$ 0.52	$ 0.48	$ 1.04	$ 0.96
Date of dividend paid									Jul. 15, 2015
Date of dividend declared									Jun. 30, 2015
Record date of dividend paid									Jun. 30, 2015
Accumulated other comprehensive loss		$ (19,332)		$ (19,332)			$ (19,332)		$ (19,332)		$ (15,024)
Gain related to changes in fair value of derivative instruments		1,200					3,308		(28)
Cumulative currency translation adjustment on assets and liabilities denominated in foreign currencies		20,500		20,500			20,500		20,500
CT Legacy Partners' total equity		21,200		21,200			21,200		21,200
Equity interests owned by Blackstone Mortgage Trust, Inc.		8,800		8,800			8,800		8,800
Non-controlling interests in CT Legacy Partners		$ 12,400		$ 12,400			$ 12,400		12,400
Subsequent Events [Member]
Class of Stock [Line Items]
Dividends paid	$ 48,500
Dividends paid per common stock	$ 0.52
Installment 1 Fiscal Year 2015 [Member]
Class of Stock [Line Items]
Dividends paid									$ 78,900	$ 42,100
Class A Common Stock [Member]
Class of Stock [Line Items]
Common stock, shares authorized		200,000,000		200,000,000			200,000,000		200,000,000		200,000,000
Common stock, shares issued		93,229,986		93,229,986			93,229,986		93,229,986		58,269,889		25,875,000
Common stock, shares issued under dividend reinvestment component									273
Aggregate sales price			$ 200,000
Number of shares sold during period				11,500,000	280,025	23,000,000			34,780,025
Net proceeds from issuance of common stock				$ 337,935	$ 7,895	$ 683,727			$ 1,029,557
Class A Common Stock [Member] | ATM Agreements [Member]
Class of Stock [Line Items]
Number of shares sold during period									280,025
Net proceeds from issuance of common stock									$ 7,900
Aggregate sales price remaining available		$ 188,600		$ 188,600			$ 188,600		$ 188,600
Class A Common Stock [Member] | Direct Stock Purchase Plan [Member]
Class of Stock [Line Items]
Common stock, shares issued		0		0			0		0
Class A Common Stock [Member] | Dividend Reinvestment and Direct Stock Purchase Plan [Member]
Class of Stock [Line Items]
Reserved for issuance of class A common stock		9,999,721		9,999,721			9,999,721		9,999,721			10,000,000