Equity - Schedule of Basic and Diluted Earnings Per Share, Continuing Operations (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Earnings Per Share [Abstract]
Income from continuing operations	$ 100,494	$ 25,424	$ 282,213
Net income attributable to non-controlling interests	(10,449)	(10,392)	(98,780)
Income from continuing operations attributable to Blackstone Mortgage Trust, Inc.	$ 90,045	$ 15,032	$ 183,433
Weighted-average shares outstanding	48,394,478	18,520,052	2,345,943
Warrants and options outstanding for the purchase of class A common stock			129,351
Weighted-average shares outstanding, diluted	48,394,478	18,520,052	2,475,294
Per share amount, basic	$ 1.86	$ 0.81	$ 78.19
Per share amount, diluted	$ 1.86	$ 0.81	$ 74.16

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Amount of income (loss) from continuing operations attributable to the parent. Also defined as revenue less expenses and taxes from ongoing operations before extraordinary items but after deduction of those portions of income or loss from continuing operations that are allocable to noncontrolling interests.

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This element represents the income or loss from continuing operations attributable to the economic entity which may also be defined as revenue less expenses and taxes from ongoing operations before extraordinary items, and noncontrolling interest.

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The amount of net income (loss) from continuing operations per each share of common stock or unit outstanding during the reporting period.

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The amount of net income (loss) derived from continuing operations during the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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Amount of Net Income (Loss) attributable to noncontrolling interest.

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The sum of dilutive potential common shares or units used in the calculation of the diluted per-share or per-unit computation.

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The average number of shares or units issued and outstanding that are used in calculating diluted EPS or earnings per unit (EPU), determined based on the timing of issuance of shares or units in the period.

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Number of [basic] shares or units, after adjustment for contingently issuable shares or units and other shares or units not deemed outstanding, determined by relating the portion of time within a reporting period that common shares or units have been outstanding to the total time in that period.

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