v2.4.1.9

Fair Value Option - Additional Information (Detail) (USD $) | Mar. 31, 2015 | Dec. 31, 2014
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Fair Value, Option, Quantitative Disclosures [Line Items] |  |  |  |
Fair value of financial instruments more than one day past due | $ 2,875,000 bx_FairValueOfFinancialInstrumentsMoreThanOneDayPastDue | [1] | $ 4,369,000 bx_FairValueOfFinancialInstrumentsMoreThanOneDayPastDue | [1]
Corporate Bonds |  |  |  |
Fair Value, Option, Quantitative Disclosures [Line Items] |  |  |  |
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 |
Fair value of financial instruments with non-accrual status | 0 bx_InvestmentSecuritiesFairValueDisclosureOnNonaccrualStatus / us-gaap_InvestmentTypeAxis = us-gaap_CorporateBondSecuritiesMember |  | 0 bx_InvestmentSecuritiesFairValueDisclosureOnNonaccrualStatus / us-gaap_InvestmentTypeAxis = us-gaap_CorporateBondSecuritiesMember |
Loans And Receivables |  |  |  |
Fair Value, Option, Quantitative Disclosures [Line Items] |  |  |  |
Fair value of financial instruments more than one day past due |  |  | 0 bx_FairValueOfFinancialInstrumentsMoreThanOneDayPastDue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = bx_LoansAndReceivablesMember |
Fair value of financial instruments with non-accrual status |  |  | $ 0 bx_InvestmentSecuritiesFairValueDisclosureOnNonaccrualStatus / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = bx_LoansAndReceivablesMember |
[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 Investment Securities Fair Value Disclosure On Nonaccrual Status + References No definition available. + Details
Name: | bx_InvestmentSecuritiesFairValueDisclosureOnNonaccrualStatus
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: |