Financial Instruments Fair Value Table (Details) - USD ($) $ in Millions		Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2015
Cash and Cash Equivalents, at Carrying Value		$ 113	$ 94	$ 69
Long-term Debt		2,295	2,263
Project Debt		404	175
Accounts Receivable, Net, Noncurrent		1	2
Estimate of Fair Value Measurement [Member]
Assets, Fair Value Disclosure		245	277
Financial and Nonfinancial Liabilities, Fair Value Disclosure		27	14
Accounts Receivable, Fair Value Disclosure	[1]	300	314
Long-term Debt		2,341	2,244
Project Debt		412	183
Reported Value Measurement [Member]
Accounts Receivable, Fair Value Disclosure	[1]	300	314
Long-term Debt		2,295	2,263
Project Debt		404	175
Fair Value, Inputs, Level 1 [Member] | Estimate of Fair Value Measurement [Member]
Cash and Cash Equivalents, at Carrying Value		113	94
Restricted Cash and Investments		119	160
Fair Value, Inputs, Level 1 [Member] | Interest-bearing Deposits [Member] | Estimate of Fair Value Measurement [Member]
Certificates of Deposit, at Carrying Value		109	89
Restricted Cash and Investments		18	9
Fair Value, Inputs, Level 1 [Member] | Money Market Funds [Member] | Estimate of Fair Value Measurement [Member]
Money Market Funds, at Carrying Value		4	5
Restricted Cash and Investments		38	66
Fair Value, Inputs, Level 1 [Member] | US Treasury and Government [Member] | Estimate of Fair Value Measurement [Member]
Restricted Cash and Investments	[2]	15	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		46	59
Fair Value, Inputs, Level 1 [Member] | Other Investments [Member] | Estimate of Fair Value Measurement [Member]
Restricted Cash and Investments		2	8
Fair Value, Inputs, Level 1 [Member] | Mutual And Bond Funds [Member] | Estimate of Fair Value Measurement [Member]
Investments, Fair Value Disclosure	[3]	2	2
Energy Hedges [Member] | Fair Value, Inputs, Level 2 [Member] | Estimate of Fair Value Measurement [Member]
Derivative Asset		11	21
Interest Rate Swap [Member] | Fair Value, Inputs, Level 2 [Member]
Derivative Liability, Fair Value, Gross Liability		$ 27	$ 14

[1] Includes $1 million and $2 million of noncurrent receivables recorded in "Other assets" in the condensed consolidated balance sheets as September 30, 2016 and December 31, 2015

[2] 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

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