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Fair Value of Financial Investments and Liabilities - Additional Information (Detail) (USD $) |  |  | 3 Months Ended | 6 Months Ended
--- | --- | --- | --- | ---
Jun. 30, 2013 | Dec. 31, 2012 | Jun. 30, 2012 Assets at Fair Value (Level 3) [Member] | Jun. 30, 2012 Assets at Fair Value (Level 3) [Member]
Investment Holdings [Line Items] |  |  |  |
Aggregate carrying value of remaining private equity funds |  |  | $ 4,000,000 | $ 4,000,000
Impairment charges - private equity funds |  |  | 2,000,000 | 2,000,000
Estimated market value of long-term and short-term debt | 3,900,000,000 | 4,000,000,000 |  |
Aggregate carrying value of long-term and short-term debt | $ 3,600,000,000 | $ 3,600,000,000 |  |

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