v2.4.1.9

Realized and Net Change in Unrealized Gains (Losses) on Investments (Detail) (USD $) In Thousands, unless otherwise specified | 3 Months Ended
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Mar. 31, 2015 | Mar. 31, 2014
Gain (Loss) on Investments [Line Items] |  |
Realized Gains | $ 22 bx_RealizedGainsLossesOnOtherInvestments | $ 6,307 bx_RealizedGainsLossesOnOtherInvestments
Net Change in Unrealized Gains (Losses) | 371 bx_NetChangeInUnrealizedGainsLossesOnOtherInvestments | (6,504)bx_NetChangeInUnrealizedGainsLossesOnOtherInvestments
Total realized and net change in unrealized gains (losses) | $ 393 us-gaap_GainLossOnSaleOfOtherInvestments | $ (197)us-gaap_GainLossOnSaleOfOtherInvestments

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- Definition Net Change In Unrealized Gains Losses On Other Investments + References No definition available. + Details
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Namespace Prefix: | bx_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Realized Gains Losses On Other Investments + References No definition available. + Details
Name: | bx_RealizedGainsLossesOnOtherInvestments
Namespace Prefix: | bx_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition The difference between the book value and the sale price of other nonspecific investments. This element is used when other, more specific, elements are not appropriate. This element refers to the gain (loss) included in earnings. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 + Details
Name: | us-gaap_GainLossOnSaleOfOtherInvestments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Details
Name: | us-gaap_ScheduleOfGainLossOnInvestmentsIncludingMarketableSecuritiesAndInvestmentsHeldAtCostIncomeStatementReportedAmountsSummaryLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration