Financial and Other Derivative Instruments (Netting Offsets of Derivative Assets and Liabilities) (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Offsetting Assets [Line Items]
Derivative Assets	$ 685	$ 711
Gross Amounts Offset in the Consolidated Statements of Financial Position	(523)	(528)
Net Amounts of Assets Presented in the Consolidated Statements of Financial Position	162	183
Gross Amounts of Recognized (Liabilities)	(763)	(705)
Gross Amounts Offset in the Consolidated Statements of Financial Position	600	562
Net Amounts of (Liabilities) Presented in the Consolidated Statements of Financial Position	(163)	(143)
Natural Gas
Offsetting Assets [Line Items]
Derivative Assets	298	387
Gross Amounts Offset in the Consolidated Statements of Financial Position	(216)	(285)
Net Amounts of Assets Presented in the Consolidated Statements of Financial Position	82	102
Gross Amounts of Recognized (Liabilities)	(350)	(383)
Gross Amounts Offset in the Consolidated Statements of Financial Position	258	294
Net Amounts of (Liabilities) Presented in the Consolidated Statements of Financial Position	(92)	(89)
Electricity
Offsetting Assets [Line Items]
Derivative Assets	377	307
Gross Amounts Offset in the Consolidated Statements of Financial Position	(299)	(232)
Net Amounts of Assets Presented in the Consolidated Statements of Financial Position	78	75
Gross Amounts of Recognized (Liabilities)	(396)	(305)
Gross Amounts Offset in the Consolidated Statements of Financial Position	327	253
Net Amounts of (Liabilities) Presented in the Consolidated Statements of Financial Position	(69)	(52)
Other
Offsetting Assets [Line Items]
Derivative Assets	4	5
Gross Amounts Offset in the Consolidated Statements of Financial Position	(3)	(2)
Net Amounts of Assets Presented in the Consolidated Statements of Financial Position	1	3
Gross Amounts of Recognized (Liabilities)	(13)	(10)
Gross Amounts Offset in the Consolidated Statements of Financial Position	12	8
Net Amounts of (Liabilities) Presented in the Consolidated Statements of Financial Position	(1)	(2)
Foreign currency exchange contracts
Offsetting Assets [Line Items]
Derivative Assets	6	12
Gross Amounts Offset in the Consolidated Statements of Financial Position	(5)	(9)
Net Amounts of Assets Presented in the Consolidated Statements of Financial Position	1	3
Gross Amounts of Recognized (Liabilities)	(4)	(7)
Gross Amounts Offset in the Consolidated Statements of Financial Position	3	7
Net Amounts of (Liabilities) Presented in the Consolidated Statements of Financial Position	$ (1)	$ 0