Equity - Additional Information (Detail) (USD $)	1 Months Ended	12 Months Ended			0 Months Ended	1 Months Ended
	Dec. 31, 2014	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	May 09, 2014	Sep. 30, 2014	Apr. 30, 2014	Jan. 31, 2014	Dec. 31, 2011	May 31, 2013	May 06, 2013	Mar. 25, 2014
Class of Stock [Line Items]
Increase in total equity		$ 744,100,000
Total equity	1,500,886,000	1,500,886,000	756,750,000	153,453,000					(128,939,000)
Total stock, shares authorized	200,000,000	200,000,000
Common stock, shares authorized	100,000,000	100,000,000	100,000,000
Preferred stock, shares authorized	100,000,000	100,000,000
Preferred stock issued	0	0
Preferred stock outstanding	0	0
Plan adoption date		March 25, 2014
Common stock issued	58,269,889	58,269,889	29,501,651
Net proceeds from issuance of common stock	2,800,000	766,138,000	633,807,000	10,000,000
Sale of common stock sales price	583,000	583,000	295,000
Common stock, shares outstanding	58,269,889	58,269,889	29,501,651
Dividends paid		84,238,000	7,776,000	48,960,000
Dividends per common stock		$ 1.98	$ 0.72	$ 20.00
Accumulated other comprehensive (loss) income	(15,024,000)	(15,024,000)	798,000
Loss on foreign currency translation		15,800,000
CT Legacy Partners' total equity	60,800,000	60,800,000
Equity interests owned by Blackstone Mortgage Trust, Inc.	25,300,000	25,300,000
Non-controlling interests in CT Legacy Partners	35,500,000	35,500,000
Installment 1 Fiscal Year 2014 [Member]
Class of Stock [Line Items]
Dividends paid		101,300,000	21,100,000	49,800,000
Class A Common Stock [Member]
Class of Stock [Line Items]
Common stock issued										25,875,000
Aggregate sales price					200,000,000
Net proceeds from issuance of common stock	2,758,000	766,138,000				252,530,000	254,758,000	256,092,000
Common stock, shares outstanding											2,926,651,000,000
Reverse stock split ratio		0.1
Description reverse stock split		On April 26, 2013, our board of directors approved a one-for-ten reverse stock split of our class A common stock which we effected on May 6, 2013.
Reclassification of common stock to additional paid-in capital		263,000,000,000
Class A Common Stock [Member] | ATM Agreements [Member]
Class of Stock [Line Items]
Common stock issued	100,000	100,000
Net proceeds from issuance of common stock		2,815,000
Sale of common stock sales price	$ 197,200,000	$ 197,200,000
Class A Common Stock [Member] | Director [Member]
Class of Stock [Line Items]
Shares issued for deferred stock units upon resignation		2,851
Class A Common Stock [Member] | Direct Stock Purchase Plan [Member]
Class of Stock [Line Items]
Common stock issued	0	0
Class A Common Stock [Member] | Dividend Reinvestment Plan [Member]
Class of Stock [Line Items]
Common stock issued	6	6
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	9,999,994	9,999,994										10,000,000