Financial and Other Derivative Instruments Financial and Other Derivative Instruments (Netting Offsets of Derivative Assets and Liabilities) (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Offsetting Assets [Line Items]
Gross Amounts of Recognized Assets	$ 586	$ 1,178
Gross Amounts Offset in the Consolidated Statements of Financial Position	(448)	(1,006)
Net Amounts of Assets Presented in the Consolidated Statements of Financial Position	138	172
Gross Amounts of Recognized (Liabilities)	(551)	(1,110)
Gross Amounts Offset in the Consolidated Statements of Financial Position	468	1,025
Net Amounts of (Liabilities) Presented in the Consolidated Statements of Financial Position	(83)	(85)
Natural Gas
Offsetting Assets [Line Items]
Gross Amounts of Recognized Assets	303	787
Gross Amounts Offset in the Consolidated Statements of Financial Position	(244)	(681)
Net Amounts of Assets Presented in the Consolidated Statements of Financial Position	59	106
Gross Amounts of Recognized (Liabilities)	(277)	(718)
Gross Amounts Offset in the Consolidated Statements of Financial Position	249	679
Net Amounts of (Liabilities) Presented in the Consolidated Statements of Financial Position	(28)	(39)
Electricity
Offsetting Assets [Line Items]
Gross Amounts of Recognized Assets	251	342
Gross Amounts Offset in the Consolidated Statements of Financial Position	(179)	(280)
Net Amounts of Assets Presented in the Consolidated Statements of Financial Position	72	62
Gross Amounts of Recognized (Liabilities)	(244)	(342)
Gross Amounts Offset in the Consolidated Statements of Financial Position	190	298
Net Amounts of (Liabilities) Presented in the Consolidated Statements of Financial Position	(54)	(44)
Other
Offsetting Assets [Line Items]
Gross Amounts of Recognized Assets	22	45
Gross Amounts Offset in the Consolidated Statements of Financial Position	(17)	(42)
Net Amounts of Assets Presented in the Consolidated Statements of Financial Position	5	3
Gross Amounts of Recognized (Liabilities)	(23)	(45)
Gross Amounts Offset in the Consolidated Statements of Financial Position	22	45
Net Amounts of (Liabilities) Presented in the Consolidated Statements of Financial Position	(1)	0
Other derivative contract
Offsetting Assets [Line Items]
Gross Amounts of Recognized Assets	10	4
Gross Amounts Offset in the Consolidated Statements of Financial Position	(8)	(3)
Net Amounts of Assets Presented in the Consolidated Statements of Financial Position	2	1
Gross Amounts of Recognized (Liabilities)	(7)	(5)
Gross Amounts Offset in the Consolidated Statements of Financial Position	7	3
Net Amounts of (Liabilities) Presented in the Consolidated Statements of Financial Position	$ 0	$ (2)