Stockholders' Equity (Details 3) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Net Income (Loss) per Share of Common Stock
Net (loss) income attributable to Balckstone Mortgage Trust, Inc.	$ (3,115)	$ 66,553
Weighted average shares outstanding, basic	3,016,425	2,283,741
Warrants and options outstanding for the purchase of class A common stock		146,406
Weighted average shares outstanding, diluted	3,016,425	2,430,147
Per share amount, basic	$ (1.03)	$ 29.14
Per share amount, diluted	$ (1.03)	$ 27.39