Financial Instruments Fair Value Table (Details) - USD ($) $ in Millions		Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015
Cash and Cash Equivalents, at Carrying Value		$ 106	$ 94	$ 63
Long-term Debt		2,343	2,263
Project Debt		207	175
Accounts Receivable, Net, Noncurrent		2	2
Estimate of Fair Value Measurement [Member]
Assets, Fair Value Disclosure		275	277
Financial and Nonfinancial Liabilities, Fair Value Disclosure		22	14
Accounts Receivable, Fair Value Disclosure	[1]	287	314
Long-term Debt		2,357	2,266
Project Debt		215	209
Reported Value Measurement [Member]
Accounts Receivable, Fair Value Disclosure	[1]	287	314
Long-term Debt		2,343	2,263
Project Debt		207	175
Fair Value, Inputs, Level 1 [Member] | Estimate of Fair Value Measurement [Member]
Cash and Cash Equivalents, at Carrying Value		106	94
Restricted Cash and Investments		142	158
Fair Value, Inputs, Level 1 [Member] | Interest-bearing Deposits [Member] | Estimate of Fair Value Measurement [Member]
Certificates of Deposit, at Carrying Value		101	89
Restricted Cash and Investments		9	7
Other Restricted Assets	[2]	2	2
Fair Value, Inputs, Level 1 [Member] | Money Market Funds [Member] | Estimate of Fair Value Measurement [Member]
Money Market Funds, at Carrying Value		5	5
Restricted Cash and Investments		51	66
Fair Value, Inputs, Level 1 [Member] | US Treasury and Government [Member] | Estimate of Fair Value Measurement [Member]
Restricted Cash and Investments	[3]	17	18
Fair Value, Inputs, Level 1 [Member] | US States and Political Subdivisions Debt Securities [Member] | Estimate of Fair Value Measurement [Member]
Restricted Cash and Investments		59	59
Fair Value, Inputs, Level 1 [Member] | Other Investments [Member] | Estimate of Fair Value Measurement [Member]
Restricted Cash and Investments		6	8
Fair Value, Inputs, Level 1 [Member] | Mutual And Bond Funds [Member] | Estimate of Fair Value Measurement [Member]
Investments, Fair Value Disclosure	[2],[4]	2	2
Fair Value, Inputs, Level 2 [Member] | Estimate of Fair Value Measurement [Member]
Foreign Currency Contract, Asset, Fair Value Disclosure		1	0
Fair Value, Inputs, Level 2 [Member] | Energy Hedges [Member] | Estimate of Fair Value Measurement [Member]
Derivative Asset, Fair Value, Amount Not Offset Against Collateral		22	21
Fair Value, Inputs, Level 2 [Member] | Interest Rate Swap [Member] | Estimate of Fair Value Measurement [Member]
Derivative Liability		$ 22	$ 14

[1] Includes $2 million and $2 million of noncurrent receivables recorded in "Other assets" in the condensed consolidated balance sheets as March 31, 2016 and December 31, 2015

[2] Included in other noncurrent assets in the condensed consolidated balance sheets

[3] 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] Included in prepaid expenses and other current assets in the condensed consolidated balance sheets