Equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands				1 Months Ended		3 Months Ended		9 Months Ended
	Oct. 15, 2018	Sep. 14, 2018	May 09, 2014	Aug. 31, 2018	May 31, 2013	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	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
Number of shares sold during period					25,875,000			11,484,414	971
Net proceeds from issuance of class A common stock								$ 369,787	$ 31
Dividends paid	$ 74,200
Dividends declared per share of common stock		$ 0.62				$ 0.62	$ 0.62	$ 1.86	$ 1.86
Dividends paid per common stock	$ 0.62
Date of dividend paid								Oct. 15, 2018
Date of dividend declared								Sep. 14, 2018
Record date of dividend paid								Sep. 28, 2018
Accumulated other comprehensive loss						$ (32,849)		$ (32,849)		$ (29,706)
Net realized and unrealized gains related to changes in fair value of derivative instruments						54,300		54,300		30,600
Cumulative unrealized currency translation adjustment on assets and liabilities denominated in foreign currencies						87,100		87,100		60,300
Multifamily [Member] | Joint Venture [Member]
Class of Stock [Line Items]
Total equity						48,100		48,100		42,300
Equity interests owned by Blackstone Mortgage Trust, Inc.						40,900		40,900		36,000
Non-controlling interests						$ 7,200		$ 7,200		$ 6,300
Dividend Reinvestment and Direct Stock Purchase Plan [Member]
Class of Stock [Line Items]
Reserved for issuance of class A common stock						9,995,647		9,995,647			10,000,000
Class A Common Stock [Member]
Class of Stock [Line Items]
Common stock, shares issued under dividend reinvestment program						403	428	1,279	971
Number of shares sold during period				6,900,000				11,483,135
Net proceeds from issuance of class A common stock				$ 222,275				$ 369,787
ATM Agreements [Member] | Class A Common Stock [Member]
Class of Stock [Line Items]
Aggregate sales price			$ 200,000
Number of shares sold during period								4,583,135	0
Net proceeds from issuance of class A common stock								$ 147,512
Aggregate sales price remaining available						$ 38,600		$ 38,600