v3.3.0.814

Information for Financial Instruments on Which Fair Value Option was Elected (Detail) - USD ($) | Sep. 30, 2015 | Dec. 31, 2014
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Fair Value, Option, Quantitative Disclosures [Line Items] |  |  |
Uncollected principal balance on financial instruments that exceeded fair value total |  | $ (51,450,000) | $ (210,717,000)
Fair value of financial instruments more than one day past due | [1] | 318,000 | 4,369,000
Uncollected principal balance on financial instruments that exceeds fair value more than one day past due | [1] | (5,897,000) | (21,876,000)
Corporate Bonds |  |  |
Fair Value, Option, Quantitative Disclosures [Line Items] |  |  |
Uncollected principal balance on financial instruments that exceeded fair value total |  | (7,449,000) | (7,814,000)
Fair value of financial instruments more than one day past due |  | 0 | 0
Debt Securities |  |  |
Fair Value, Option, Quantitative Disclosures [Line Items] |  |  |
Uncollected principal balance on financial instruments that exceeded fair value total |  | (342,000) |
Corporate Loans |  |  |
Fair Value, Option, Quantitative Disclosures [Line Items] |  |  |
Uncollected principal balance on financial instruments that exceeded fair value total |  | (36,356,000) | (197,580,000)
Fair value of financial instruments more than one day past due | [1] | 318,000 | 4,369,000
Uncollected principal balance on financial instruments that exceeds fair value more than one day past due | [1] | (5,897,000) | (21,876,000)
Loans and Receivables |  |  |
Fair Value, Option, Quantitative Disclosures [Line Items] |  |  |
Uncollected principal balance on financial instruments that exceeded fair value total |  | $ (7,303,000) | (5,323,000)
Fair value of financial instruments more than one day past due |  |  | $ 0
[1] | Corporate Loans and Corporate Bonds within CLO assets are classified as past due if contractual payments are more than one day past due.

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- Definition Fair Value of Financial Instruments More Than One Day Past Due + References No definition available. + Details
Name: | bx_FairValueOfFinancialInstrumentsMoreThanOneDayPastDue
Namespace Prefix: | bx_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Uncollected principal balance on financial instruments that exceeded fair value. + References No definition available. + Details
Name: | bx_UncollectedPrincipalBalanceOnFinancialInstrumentsThatExceededFairValue
Namespace Prefix: | bx_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Uncollected Principal Balance On Financial Instruments That Exceeded Fair Value More than One Day Past Due + References No definition available. + Details
Name: | bx_UncollectedPrincipalBalanceOnFinancialInstrumentsThatExceedsFairValueMoreThanOneDayPastDue
Namespace Prefix: | bx_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_FairValueOptionQuantitativeDisclosuresLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_InvestmentTypeAxis=us-gaap_CorporateBondSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_FairValueByAssetClassAxis=us-gaap_DebtSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_CorporateDebtSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=us-gaap_LoansReceivableMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |