Fair Value Measurements - Fair Value of Assets and Liabilities Measured on Recurring Basis (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Assets
Money market funds	$ 127	$ 204
Fixed-income securities	37	36
Redeemable preferred stock	25	25
Interest rate derivatives		73
Electricity commodity derivatives	1	5
Total assets	190	343
Liabilities
Interest rate derivatives	42	74
Foreign currency derivatives	11	2
Electricity commodity derivatives	5
Total liabilities	58	76
Quoted Prices in Active Markets (Level 1) [Member]
Assets
Money market funds	127	204
Total assets	127	204
Significant Other Observable Inputs (Level 2) [Member]
Assets
Fixed-income securities	37	36
Interest rate derivatives		73
Electricity commodity derivatives	1	5
Total assets	38	114
Liabilities
Interest rate derivatives	42	74
Foreign currency derivatives	11	2
Electricity commodity derivatives	5
Total liabilities	58	76
Significant Unobservable Inputs (Level 3) [Member]
Assets
Redeemable preferred stock	25	25
Total assets	$ 25	$ 25