Financial and Other Derivative Instruments Financial and Other Derivative Instruments (Netting Offsets of Derivative Assets and Liabilities) (Details) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Offsetting Assets [Line Items]
Gross Amounts of Recognized Assets	$ 607	$ 1,178
Gross Amounts Offset in the Consolidated Statements of Financial Position	(482)	(1,006)
Net Amounts of Assets Presented in the Consolidated Statements of Financial Position	125	172
Gross Amounts of Recognized (Liabilities)	(601)	(1,110)
Gross Amounts Offset in the Consolidated Statements of Financial Position	517	1,025
Net Amounts of (Liabilities) Presented in the Consolidated Statements of Financial Position	(84)	(85)
Natural Gas
Offsetting Assets [Line Items]
Gross Amounts of Recognized Assets	251	787
Gross Amounts Offset in the Consolidated Statements of Financial Position	(198)	(681)
Net Amounts of Assets Presented in the Consolidated Statements of Financial Position	53	106
Gross Amounts of Recognized (Liabilities)	(256)	(718)
Gross Amounts Offset in the Consolidated Statements of Financial Position	215	679
Net Amounts of (Liabilities) Presented in the Consolidated Statements of Financial Position	(41)	(39)
Electricity
Offsetting Assets [Line Items]
Gross Amounts of Recognized Assets	312	342
Gross Amounts Offset in the Consolidated Statements of Financial Position	(249)	(280)
Net Amounts of Assets Presented in the Consolidated Statements of Financial Position	63	62
Gross Amounts of Recognized (Liabilities)	(305)	(342)
Gross Amounts Offset in the Consolidated Statements of Financial Position	265	298
Net Amounts of (Liabilities) Presented in the Consolidated Statements of Financial Position	(40)	(44)
Other
Offsetting Assets [Line Items]
Gross Amounts of Recognized Assets	37	45
Gross Amounts Offset in the Consolidated Statements of Financial Position	(31)	(42)
Net Amounts of Assets Presented in the Consolidated Statements of Financial Position	6	3
Gross Amounts of Recognized (Liabilities)	(34)	(45)
Gross Amounts Offset in the Consolidated Statements of Financial Position	33	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	7	4
Gross Amounts Offset in the Consolidated Statements of Financial Position	(4)	(3)
Net Amounts of Assets Presented in the Consolidated Statements of Financial Position	3	1
Gross Amounts of Recognized (Liabilities)	(6)	(5)
Gross Amounts Offset in the Consolidated Statements of Financial Position	4	3
Net Amounts of (Liabilities) Presented in the Consolidated Statements of Financial Position	$ (2)	$ (2)