Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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+ Details

Fair Value Measurements - Rating of U.S. Corporate Debt Securities (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Rating Of Available For Sale Securities [Line Items]
Securities available-for-sale	$ 116	$ 188
U.S. corporate debt securities [Member]
Rating Of Available For Sale Securities [Line Items]
Securities available-for-sale		57
U.S. corporate debt securities [Member] \| AAA to AA [Member]
Rating Of Available For Sale Securities [Line Items]
Securities available-for-sale		26
U.S. corporate debt securities [Member] \| A+ to A- [Member]
Rating Of Available For Sale Securities [Line Items]
Securities available-for-sale		25
U.S. corporate debt securities [Member] \| BBB+ to Unrated [Member]
Rating Of Available For Sale Securities [Line Items]
Securities available-for-sale		6
Level 2 [Member] \| U.S. corporate debt securities [Member] \| AAA to AA [Member]
Rating Of Available For Sale Securities [Line Items]
Securities available-for-sale		26
Level 2 [Member] \| U.S. corporate debt securities [Member] \| A+ to A- [Member]
Rating Of Available For Sale Securities [Line Items]
Securities available-for-sale		25
Level 2 [Member] \| U.S. corporate debt securities [Member] \| BBB+ to Unrated [Member]
Rating Of Available For Sale Securities [Line Items]
Securities available-for-sale		$ 6