Fair Value Measurements and Credit Concentration (Assets and Liabilities Measured on Recurring Basis Table) (Details) - USD ($) $ in Millions		Sep. 30, 2016	Dec. 31, 2015
Price risk management assets:
Gross		$ 544	$ 693
Price risk management liabilities:
Gross		398	539
Recurring
Assets
Cash and cash equivalents		1,358	141
Restricted cash and cash equivalents	[1]	46	106
Price risk management assets:
Gross		544	693
NDT funds		1,012	951
Total assets		2,963	1,897
Price risk management liabilities:
Gross		398	539
Recurring | Cash and cash equivalents
Price risk management assets:
NDT funds		5	11
Recurring | U.S. large-cap
Price risk management assets:
NDT funds		662	616
Recurring | U.S. mid/small-cap
Price risk management assets:
NDT funds		95	87
Recurring | U.S. Treasury
Price risk management assets:
NDT funds		114	98
Recurring | U.S. government sponsored agency
Price risk management assets:
NDT funds		3	6
Recurring | Municipality
Price risk management assets:
NDT funds		86	83
Recurring | Investment-grade corporate
Price risk management assets:
NDT funds		46	47
Recurring | Other
Price risk management assets:
NDT funds		0	3
Recurring | Receivables/payables, net
Price risk management assets:
NDT funds		1	0
Recurring | Auction Rate Securities
Price risk management assets:
Auction rate securities	[2]	3	6
Recurring | Level 1
Assets
Cash and cash equivalents		1,358	141
Restricted cash and cash equivalents	[1]	46	106
Price risk management assets:
Gross		0	0
NDT funds		648	601
Total assets		2,052	848
Price risk management liabilities:
Gross		0	0
Recurring | Level 1 | Cash and cash equivalents
Price risk management assets:
NDT funds		5	11
Recurring | Level 1 | U.S. large-cap
Price risk management assets:
NDT funds		490	457
Recurring | Level 1 | U.S. mid/small-cap
Price risk management assets:
NDT funds		40	37
Recurring | Level 1 | U.S. Treasury
Price risk management assets:
NDT funds		114	98
Recurring | Level 1 | Receivables/payables, net
Price risk management assets:
NDT funds		(1)	(2)
Recurring | Level 2
Price risk management assets:
Gross		494	597
NDT funds		364	350
Total assets		858	947
Price risk management liabilities:
Gross		392	497
Recurring | Level 2 | U.S. large-cap
Price risk management assets:
NDT funds		172	159
Recurring | Level 2 | U.S. mid/small-cap
Price risk management assets:
NDT funds		55	50
Recurring | Level 2 | U.S. government sponsored agency
Price risk management assets:
NDT funds		3	6
Recurring | Level 2 | Municipality
Price risk management assets:
NDT funds		86	83
Recurring | Level 2 | Investment-grade corporate
Price risk management assets:
NDT funds		46	47
Recurring | Level 2 | Other
Price risk management assets:
NDT funds		0	3
Recurring | Level 2 | Receivables/payables, net
Price risk management assets:
NDT funds		2	2
Recurring | Level 3
Price risk management assets:
Gross		50	96
Total assets		53	102
Price risk management liabilities:
Gross		6	42
Recurring | Level 3 | Auction Rate Securities
Price risk management assets:
Auction rate securities	[2]	3	6
Energy Commodities, net | Recurring
Price risk management assets:
Gross		544	693
Price risk management liabilities:
Gross		398	539
Energy Commodities, net | Recurring | Level 1
Price risk management assets:
Gross		0	0
Price risk management liabilities:
Gross		0	0
Energy Commodities, net | Recurring | Level 2
Price risk management assets:
Gross		494	597
Price risk management liabilities:
Gross		392	497
Energy Commodities, net | Recurring | Level 3
Price risk management assets:
Gross		50	96
Price risk management liabilities:
Gross		$ 6	$ 42

[1] Current portion is included in "Restricted cash and cash equivalents" and long-term portion, when applicable, is included in "Other noncurrent assets" on the Balance Sheets.

[2] Included in "Other investments" on the Balance Sheets.