Fair Value Measurements and Credit Concentration (Assets and Liabilities Measured on Recurring Basis Table) (Details) - USD ($) $ in Millions		Sep. 30, 2015	Dec. 31, 2014
Price risk management assets:
Gross		$ 802	$ 1,318
Price risk management liabilities:
Gross		684	1,217
Recurring
Assets
Cash and cash equivalents		648	352
Restricted cash and cash equivalents	[1]	83	193
Price risk management assets:
Gross		802	1,318
NDT funds		913	950
Total assets		2,452	2,821
Price risk management liabilities:
Gross		684	1,217
Recurring | Cash and cash equivalents
Price risk management assets:
NDT funds		13	19
Recurring | U.S. large-cap
Price risk management assets:
NDT funds		581	611
Recurring | U.S. mid/small-cap
Price risk management assets:
NDT funds		84	89
Recurring | U.S. Treasury
Price risk management assets:
NDT funds		94	99
Recurring | U.S. government sponsored agency
Price risk management assets:
NDT funds		7	9
Recurring | Municipality
Price risk management assets:
NDT funds		81	76
Recurring | Investment-grade corporate
Price risk management assets:
NDT funds		49	42
Recurring | Other
Price risk management assets:
NDT funds		4	3
Recurring | Receivables/payables, net
Price risk management assets:
NDT funds		0	2
Recurring | Auction Rate Securities
Price risk management assets:
Auction rate securities	[2]	6	8
Recurring | Level 1
Assets
Cash and cash equivalents		648	352
Restricted cash and cash equivalents	[1]	83	193
Price risk management assets:
Gross		0	6
NDT funds		572	609
Total assets		1,303	1,160
Price risk management liabilities:
Gross		0	5
Recurring | Level 1 | Cash and cash equivalents
Price risk management assets:
NDT funds		13	19
Recurring | Level 1 | U.S. large-cap
Price risk management assets:
NDT funds		433	454
Recurring | Level 1 | U.S. mid/small-cap
Price risk management assets:
NDT funds		35	37
Recurring | Level 1 | U.S. Treasury
Price risk management assets:
NDT funds		94	99
Recurring | Level 1 | Receivables/payables, net
Price risk management assets:
NDT funds		(3)	0
Recurring | Level 2
Price risk management assets:
Gross		739	1,171
NDT funds		341	341
Total assets		1,080	1,512
Price risk management liabilities:
Gross		626	1,182
Recurring | Level 2 | U.S. large-cap
Price risk management assets:
NDT funds		148	157
Recurring | Level 2 | U.S. mid/small-cap
Price risk management assets:
NDT funds		49	52
Recurring | Level 2 | U.S. government sponsored agency
Price risk management assets:
NDT funds		7	9
Recurring | Level 2 | Municipality
Price risk management assets:
NDT funds		81	76
Recurring | Level 2 | Investment-grade corporate
Price risk management assets:
NDT funds		49	42
Recurring | Level 2 | Other
Price risk management assets:
NDT funds		4	3
Recurring | Level 2 | Receivables/payables, net
Price risk management assets:
NDT funds		3	2
Recurring | Level 3
Price risk management assets:
Gross		63	141
Total assets		69	149
Price risk management liabilities:
Gross		58	30
Recurring | Level 3 | Auction Rate Securities
Price risk management assets:
Auction rate securities	[2]	6	8
Energy Commodities, net | Recurring
Price risk management assets:
Gross		802	1,318
Price risk management liabilities:
Gross		684	1,217
Energy Commodities, net | Recurring | Level 1
Price risk management assets:
Gross		0	6
Price risk management liabilities:
Gross		0	5
Energy Commodities, net | Recurring | Level 2
Price risk management assets:
Gross		739	1,171
Price risk management liabilities:
Gross		626	1,182
Energy Commodities, net | Recurring | Level 3
Price risk management assets:
Gross		63	141
Price risk management liabilities:
Gross		$ 58	$ 30

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

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