Fair Value Measurement of Assets and Liabilities (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Assets:
Cash and cash equivalents	$ 246		$ 232		$ 126
Restricted funds held in trust	214
Restricted funds - other	14		13
Liabilities:
Long-term Debt	2,015		1,486
Project Debt	317		680
Carrying (Reported) Amount, Fair Value Disclosure [Member]
Assets:
Accounts receivables	283	[1]	277	[1]
Liabilities:
Long-term Debt	2,015		1,486
Project Debt	317		680
Estimate of Fair Value, Fair Value Disclosure [Member]
Assets:
Accounts receivables	283	[1]	277	[1]
Liabilities:
Long-term Debt	2,081		1,470
Project Debt	329		693
Fair Value, Inputs, Level 1 [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Assets:
Cash and cash equivalents	246		232
Restricted funds held in trust	214		191
Restricted funds - other	14		13
Investments	41		34
Fair Value, Inputs, Level 1 [Member] | Estimate of Fair Value, Fair Value Disclosure [Member] | Bank deposits and certificates of deposit [Member]
Assets:
Bank deposits and certificates of deposit	240		225
Restricted funds held in trust	2		5
Restricted funds - other	5	[1],[2]	5	[1],[2]
Fair Value, Inputs, Level 1 [Member] | Estimate of Fair Value, Fair Value Disclosure [Member] | Money market funds [Member]
Assets:
Money Market Funds, at Carrying Value	6		7
Restricted funds held in trust	64		119
Restricted funds - other	8	[2]	7	[2]
Fair Value, Inputs, Level 1 [Member] | Estimate of Fair Value, Fair Value Disclosure [Member] | U.S. Treasury/Agency obligations [Member]
Assets:
Restricted funds held in trust	125	[3]	15	[3]
Investments available for sale	6	[4]	8	[4]
Fair Value, Inputs, Level 1 [Member] | Estimate of Fair Value, Fair Value Disclosure [Member] | State and municipal obligations [Member]
Assets:
Restricted funds held in trust	11		7
Fair Value, Inputs, Level 1 [Member] | Estimate of Fair Value, Fair Value Disclosure [Member] | Commercial paper/Guaranteed investment contracts/Repurchase agreements [Member]
Assets:
Restricted funds held in trust	12		45
Fair Value, Inputs, Level 1 [Member] | Estimate of Fair Value, Fair Value Disclosure [Member] | Residential mortgage-backed securities [Member]
Assets:
Restricted funds - other	1	[2]	1	[2]
Investments available for sale	11	[4]	7	[4]
Fair Value, Inputs, Level 1 [Member] | Estimate of Fair Value, Fair Value Disclosure [Member] | Mutual And Bond Funds [Member]
Assets:
Investments	2	[1]	2	[1]
Fair Value, Inputs, Level 1 [Member] | Estimate of Fair Value, Fair Value Disclosure [Member] | Other government obligations [Member]
Assets:
Investments available for sale	5	[4]	3	[4]
Fair Value, Inputs, Level 1 [Member] | Estimate of Fair Value, Fair Value Disclosure [Member] | Corporate investments [Member]
Assets:
Investments available for sale	14	[4]	13	[4]
Fair Value, Inputs, Level 1 [Member] | Estimate of Fair Value, Fair Value Disclosure [Member] | Equity Securities [Member]
Assets:
Investments available for sale	3	[2]	1	[2]
Fair Value, Inputs, Level 2 [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Assets:
Total assets:	619		520
Liabilities:
Total liabilities:	106		49
Fair Value, Inputs, Level 2 [Member] | Estimate of Fair Value, Fair Value Disclosure [Member] | Note Hedge [Member]
Assets:
Derivative Assets	104		47
Fair Value, Inputs, Level 2 [Member] | Estimate of Fair Value, Fair Value Disclosure [Member] | Energy Hedges [Member]
Assets:
Derivative Asset, Fair Value, Net	0		3
Liabilities:
Derivative Liability, Fair Value, Net	1		0
Fair Value, Inputs, Level 2 [Member] | Estimate of Fair Value, Fair Value Disclosure [Member] | Cash Conversion Option [Member]
Liabilities:
Derivative Financial Instruments, Liabilities, Fair Value Disclosure	105		49
Fair Value, Inputs, Level 2 [Member] | Estimate of Fair Value, Fair Value Disclosure [Member] | Contingent interest features of the 3.25% Notes and Debentures [Member]
Liabilities:
Derivative Financial Instruments, Liabilities, Fair Value Disclosure	$ 0		$ 0

[1] (2)Included in other noncurrent assets in the consolidated balance sheets.

[2] (3)Included in prepaid expenses and other current assets in the consolidated balance sheets.

[3] (1)The U.S. Treasury/Agency obligations in restricted funds held in trust are primarily comprised of Federal Home Loan Mortgage Corporation securities at fair value.

[4] (4)Included in investments in fixed maturities at market in the consolidated balance sheets.