Available-For-Sale Securities (Amount Of Available-For-Sale Debt Securities By Maturity Periods) (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, Less than one year	$ 65
Available-for-sale debt securities, 1-5 years	144
Available-for-sale debt securities, 6-10 years	183
Available-for-sale debt securities, 11-15 years	47
Available-for-sale debt securities, 16-20 years	11
Available-for-sale debt securities, Over 20 years	255
Total Available-for-Sale Debt Securities	705
Rabbi Trusts [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, Less than one year	0
Available-for-sale debt securities, 1-5 years	61
Available-for-sale debt securities, 6-10 years	29
Available-for-sale debt securities, 11-15 years	10
Available-for-sale debt securities, 16-20 years	5
Available-for-sale debt securities, Over 20 years	57
Total Available-for-Sale Debt Securities	$ 162

Amount of debt securities categorized neither as held-to-maturity nor trading.

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