Financial and Other Derivative Instruments (Netting Offsets of Derivative Assets and Liabilities) (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Offsetting Assets [Line Items]
Derivative Assets	$ 489	$ 711
Gross Amounts Offset in the Consolidated Statements of Financial Position, Assets	(387)	(528)
Net Amounts of Assets Presented in the Consolidated Statements of Financial Position, Assets	102	183
Derivative Liabilities	(601)	(705)
Gross Amounts Offset in the Consolidated Statements of Financial Position, Liabilities	417	562
Net Amounts of (Liabilities) Presented in the Consolidated Statements of Financial Position, Liabilities	(184)	(143)
Natural Gas
Offsetting Assets [Line Items]
Derivative Assets	264	387
Gross Amounts Offset in the Consolidated Statements of Financial Position, Assets	(209)	(285)
Net Amounts of Assets Presented in the Consolidated Statements of Financial Position, Assets	55	102
Derivative Liabilities	(361)	(383)
Gross Amounts Offset in the Consolidated Statements of Financial Position, Liabilities	218	294
Net Amounts of (Liabilities) Presented in the Consolidated Statements of Financial Position, Liabilities	(143)	(89)
Electricity
Offsetting Assets [Line Items]
Derivative Assets	213	307
Gross Amounts Offset in the Consolidated Statements of Financial Position, Assets	(169)	(232)
Net Amounts of Assets Presented in the Consolidated Statements of Financial Position, Assets	44	75
Derivative Liabilities	(226)	(305)
Gross Amounts Offset in the Consolidated Statements of Financial Position, Liabilities	185	253
Net Amounts of (Liabilities) Presented in the Consolidated Statements of Financial Position, Liabilities	(41)	(52)
Other
Offsetting Assets [Line Items]
Derivative Assets	7	5
Gross Amounts Offset in the Consolidated Statements of Financial Position, Assets	(4)	(2)
Net Amounts of Assets Presented in the Consolidated Statements of Financial Position, Assets	3	3
Derivative Liabilities	(11)	(10)
Gross Amounts Offset in the Consolidated Statements of Financial Position, Liabilities	11	8
Net Amounts of (Liabilities) Presented in the Consolidated Statements of Financial Position, Liabilities	0	(2)
Foreign currency exchange contracts
Offsetting Assets [Line Items]
Derivative Assets	5	12
Gross Amounts Offset in the Consolidated Statements of Financial Position, Assets	(5)	(9)
Net Amounts of Assets Presented in the Consolidated Statements of Financial Position, Assets	0	3
Derivative Liabilities	(3)	(7)
Gross Amounts Offset in the Consolidated Statements of Financial Position, Liabilities	3	7
Net Amounts of (Liabilities) Presented in the Consolidated Statements of Financial Position, Liabilities	$ 0	$ 0