Equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands				1 Months Ended								12 Months Ended
	Jan. 15, 2016	Dec. 14, 2015	May. 09, 2014	Jun. 30, 2015	May. 31, 2015	Apr. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Apr. 30, 2014	Jan. 31, 2014	May. 31, 2013	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Mar. 25, 2014
Class of Stock [Line Items]
Total stock, shares authorized												300,000,000
Preferred stock, shares authorized												100,000,000
Preferred stock issued												0
Preferred stock outstanding												0
Plan adoption date												Mar. 25, 2014
Net proceeds from issuance of common stock												$ 1,029,540	$ 766,138	$ 633,807
Dividends declared per share of common stock		$ 0.62										$ 2.28	$ 1.98	$ 0.72
Date of dividend paid												Jan. 15, 2016
Date of dividend declared												Dec. 14, 2015
Record date of dividend paid												Dec. 31, 2015
Accumulated other comprehensive loss							$ 15,024					$ 32,758	$ 15,024
Gain related to changes in fair value of derivative instruments												25,000
Cumulative currency translation adjustment on assets and liabilities denominated in foreign currencies												57,800
CT Legacy Partners' total equity							60,800					22,500	60,800
Equity interests owned by Blackstone Mortgage Trust, Inc.							25,300					9,400	25,300
Non-controlling interests in CT Legacy Partners							$ 35,500					$ 13,100	$ 35,500
Subsequent Events [Member]
Class of Stock [Line Items]
Dividends paid	$ 58,100
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
Common stock, shares issued under dividend reinvestment component												597	6
Number of shares sold during period				11,500,000	280,025	23,000,000	100,000	9,200,000	9,200,000	9,775,000	25,875,000	34,780,025	28,275,000
Net proceeds from issuance of common stock				$ 337,918	$ 7,895	$ 683,722	$ 2,758	$ 252,530	$ 254,758	$ 256,092		$ 1,029,535	$ 766,138
Class A Common Stock [Member] | ATM Agreements [Member]
Class of Stock [Line Items]
Aggregate sales price			$ 200,000
Number of shares sold during period												280,025	100,000
Net proceeds from issuance of common stock												$ 7,900	$ 2,800
Aggregate sales price remaining available												$ 188,600
Dividend Reinvestment and Direct Stock Purchase Plan [Member] | Class A Common Stock [Member]
Class of Stock [Line Items]
Reserved for issuance of class A common stock												9,999,397			10,000,000