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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, 2013
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Dec. 31, 2012
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| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ||||||||
| Financial investments, at fair value | $ 218 | [1] | $ 223 | [1] | ||||
| Default fund investments | 191 | [2] | 175 | [2] | ||||
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Foreign Government Debt Securities [Member]
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| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ||||||||
| Financial investments, at fair value | 195 | 201 | ||||||
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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 | 141 | 134 | ||||||
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Available-for-sale Securities [Member]
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| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ||||||||
| Financial investments, at fair value | 23 | 22 | ||||||
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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 fund investments | 118 | |||||||
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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 fund investments | $ 73 | |||||||
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