Equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands					1 Months Ended	3 Months Ended		9 Months Ended
	Oct. 15, 2015	Sep. 30, 2015	Sep. 14, 2015	May. 09, 2014	May. 31, 2015	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 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
Preferred stock, shares authorized		100,000,000				100,000,000		100,000,000
Preferred stock issued		0				0		0
Preferred stock outstanding		0				0		0
Plan adoption date								Mar. 25, 2014
Net proceeds from issuance of common stock								$ 1,029,535	$ 763,407
Dividends paid								$ 109,178	$ 55,399
Dividends declared per common stock			$ 0.62			$ 0.62	$ 0.50	$ 1.66	$ 1.46
Date of dividend paid								Oct. 15, 2015
Date of dividend declared								Sep. 14, 2015
Record date of dividend paid								Sep. 30, 2015
Accumulated other comprehensive loss		$ 27,588				$ 27,588		$ 27,588		$ 15,024
Gain related to changes in fair value of derivative instruments		16,100				14,891		14,920
Cumulative currency translation adjustment on assets and liabilities denominated in foreign currencies		43,600				43,600		43,600
CT Legacy Partners' total equity		22,300				22,300		22,300		60,800
Equity interests owned by Blackstone Mortgage Trust, Inc.		9,300				9,300		9,300		25,300
Non-controlling interests in CT Legacy Partners		$ 13,000				$ 13,000		$ 13,000		$ 35,500
Subsequent Events [Member]
Class of Stock [Line Items]
Dividends paid	$ 57,800
Dividends paid per common stock	$ 0.62
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
Common stock, shares issued		93,212,863				93,212,863		93,212,863		58,269,889		25,875,000
Common stock, shares issued under dividend reinvestment component						145	2	418	3
Aggregate sales price				$ 200,000
Number of shares sold during period								34,780,443	28,175,003
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
Class A Common Stock [Member] | Direct Stock Purchase Plan [Member]
Class of Stock [Line Items]
Common stock, shares issued		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		9,999,576				9,999,576		9,999,576			10,000,000