Equity - Additional Information (Detail) - USD ($)				1 Months Ended	3 Months Ended		9 Months Ended
	Oct. 14, 2016	Sep. 13, 2016	May 09, 2014	May 31, 2013	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015	Mar. 25, 2014
Class of Stock [Line Items]
Total stock, shares authorized					300,000,000		300,000,000
Common stock, shares authorized					200,000,000		200,000,000		200,000,000
Preferred stock, shares authorized					100,000,000		100,000,000
Preferred stock issued					0		0
Preferred stock outstanding					0		0
Plan adoption date					Mar. 25, 2014
Common stock, shares issued under dividend reinvestment component					262	145	812	418
Number of shares sold during period				25,875,000			812	34,780,443
Dividends declared per share of common stock		$ 0.62			$ 0.62	$ 0.62	$ 1.86	$ 1.66
Date of dividend paid							Oct. 14, 2016
Date of dividend declared							Sep. 13, 2016
Record date of dividend paid							Sep. 30, 2016
Accumulated other comprehensive loss					$ 46,389,000		$ 46,389,000		$ 32,758,000
Realized and unrealized gains related to changes in fair value of derivative instruments					36,900,000		36,900,000		25,000,000
Cumulative unrealized currency translation adjustment on assets and liabilities denominated in foreign currencies					83,300,000		83,300,000		57,800,000
Equity interests owned by Blackstone Mortgage Trust, Inc.									9,400,000
Non-controlling interests in CT Legacy Partners									13,100,000
CT Legacy Partners [Member]
Class of Stock [Line Items]
CT Legacy Partners' total equity					1,900,000		1,900,000		$ 22,500,000
Equity interests owned by Blackstone Mortgage Trust, Inc.					786,000		786,000
Non-controlling interests in CT Legacy Partners					1,100,000		$ 1,100,000
Subsequent Events [Member]
Class of Stock [Line Items]
Dividends paid	$ 58,200,000
Dividends paid per common stock	$ 0.62
ATM Agreements [Member]
Class of Stock [Line Items]
Aggregate sales price			$ 200,000,000
Number of shares sold during period							0	0
Aggregate sales price remaining available					$ 188,600,000		$ 188,600,000
Dividend Reinvestment and Direct Stock Purchase Plan [Member]
Class of Stock [Line Items]
Reserved for issuance of class A common stock					9,998,585		9,998,585			10,000,000