Fair Value of Assets and Liabilities Measured on Recurring Basis (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Assets:
Cash equivalents	$ 270	$ 120
Available-for-sale securities	164	179
Interest rate derivatives		73
Electricity commodity derivatives	1	5
Total assets	435	377
Liabilities:
Electricity commodity derivatives	4
Foreign exchange derivatives	16	2
Interest rate derivatives	42	74
Total liabilities	62	76
Quoted Prices in Active Markets (Level 1) [Member]
Assets:
Cash equivalents	270	120
Available-for-sale securities	139	154
Total assets	409	274
Significant Other Observable Inputs (Level 2) [Member]
Assets:
Interest rate derivatives		73
Electricity commodity derivatives	1	5
Total assets	1	78
Liabilities:
Electricity commodity derivatives	4
Foreign exchange derivatives	16	2
Interest rate derivatives	42	74
Total liabilities	62	76
Significant Unobservable Inputs (Level 3) [Member]
Assets:
Available-for-sale securities	25	25
Total assets	$ 25	$ 25