v2.4.1.9

Information for Financial Instruments on Which Fair Value Option was Elected (Detail) (USD $) | Mar. 31, 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 | $ (93,521,000)bx_UncollectedPrincipalBalanceOnFinancialInstrumentsThatExceededFairValue |  | $ (210,717,000)bx_UncollectedPrincipalBalanceOnFinancialInstrumentsThatExceededFairValue |
Fair value of financial instruments more than one day past due | 2,875,000 bx_FairValueOfFinancialInstrumentsMoreThanOneDayPastDue | [1] | 4,369,000 bx_FairValueOfFinancialInstrumentsMoreThanOneDayPastDue | [1]
Uncollected principal balance on financial instruments that exceeds fair value more than one day past due | (25,514,000)bx_UncollectedPrincipalBalanceOnFinancialInstrumentsThatExceedsFairValueMoreThanOneDayPastDue | [1] | (21,876,000)bx_UncollectedPrincipalBalanceOnFinancialInstrumentsThatExceedsFairValueMoreThanOneDayPastDue | [1]
Corporate Bonds |  |  |  |
Fair Value, Option, Quantitative Disclosures [Line Items] |  |  |  |
Uncollected principal balance on financial instruments that exceeded fair value total | (2,324,000)bx_UncollectedPrincipalBalanceOnFinancialInstrumentsThatExceededFairValue / us-gaap_InvestmentTypeAxis = us-gaap_CorporateBondSecuritiesMember |  | (7,814,000)bx_UncollectedPrincipalBalanceOnFinancialInstrumentsThatExceededFairValue / us-gaap_InvestmentTypeAxis = us-gaap_CorporateBondSecuritiesMember |
Fair value of financial instruments more than one day past due | 0 bx_FairValueOfFinancialInstrumentsMoreThanOneDayPastDue / us-gaap_InvestmentTypeAxis = us-gaap_CorporateBondSecuritiesMember |  | 0 bx_FairValueOfFinancialInstrumentsMoreThanOneDayPastDue / us-gaap_InvestmentTypeAxis = us-gaap_CorporateBondSecuritiesMember |
Loans And Receivables |  |  |  |
Fair Value, Option, Quantitative Disclosures [Line Items] |  |  |  |
Uncollected principal balance on financial instruments that exceeded fair value total | (7,273,000)bx_UncollectedPrincipalBalanceOnFinancialInstrumentsThatExceededFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = bx_LoansAndReceivablesMember |  | (5,323,000)bx_UncollectedPrincipalBalanceOnFinancialInstrumentsThatExceededFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = bx_LoansAndReceivablesMember |
Fair value of financial instruments more than one day past due |  |  | 0 bx_FairValueOfFinancialInstrumentsMoreThanOneDayPastDue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = bx_LoansAndReceivablesMember |
Corporate Loans |  |  |  |
Fair Value, Option, Quantitative Disclosures [Line Items] |  |  |  |
Uncollected principal balance on financial instruments that exceeded fair value total | (83,924,000)bx_UncollectedPrincipalBalanceOnFinancialInstrumentsThatExceededFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |  | (197,580,000)bx_UncollectedPrincipalBalanceOnFinancialInstrumentsThatExceededFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |
Fair value of financial instruments more than one day past due | 2,875,000 bx_FairValueOfFinancialInstrumentsMoreThanOneDayPastDue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember | [1] | 4,369,000 bx_FairValueOfFinancialInstrumentsMoreThanOneDayPastDue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember | [1]
Uncollected principal balance on financial instruments that exceeds fair value more than one day past due | $ (25,514,000)bx_UncollectedPrincipalBalanceOnFinancialInstrumentsThatExceedsFairValueMoreThanOneDayPastDue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember | [1] | $ (21,876,000)bx_UncollectedPrincipalBalanceOnFinancialInstrumentsThatExceedsFairValueMoreThanOneDayPastDue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember | [1]
[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

X
- 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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- 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: |

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

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