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Fair Value of Financial Instruments (Schedule of Financial Assets and Liabilities Measured at Fair Value on Recurring Basis Additional Information) (Details) (USD $)
In Millions, unless otherwise specified |
Mar. 31, 2014
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Dec. 31, 2013
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| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ||||||
| Financial investments, at fair value | $ 197 | [1] | $ 189 | [1] | ||
| Default funds and margin deposits | 2,253 | 1,961 | ||||
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Restricted assets [Member]
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| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ||||||
| Financial investments, at fair value | 177 | 167 | ||||
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Reverse Repurchase Agreements [Member]
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| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ||||||
| Default funds and margin deposits | 1,501 | 1,093 | ||||
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US Government Debt Securities [Member]
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| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ||||||
| Default funds and margin deposits | $ 710 | $ 774 | ||||
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