Financial and Other Derivative Instruments (Netting Offsets of Derivative Assets and Liabilites) (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Offsetting Assets [Line Items]
Derivative Assets	$ 711	$ 1,178
Gross Amounts Offset in the Consolidated Statements of Financial Position	(528)	(1,006)
Net Amounts of Assets (Liabilities) Presented in the Consolidated Statements of Financial Position	183	172
Gross Amounts of Recognized Assets (Liabilities)	(705)	(1,110)
Gross Amounts Offset in the Consolidated Statements of Financial Position	562	1,025
Net Amounts of Assets (Liabilities) Presented in the Consolidated Statements of Financial Position	(143)	(85)
Natural Gas
Offsetting Assets [Line Items]
Derivative Assets	387	787
Gross Amounts Offset in the Consolidated Statements of Financial Position	(285)	(681)
Net Amounts of Assets (Liabilities) Presented in the Consolidated Statements of Financial Position	102	106
Gross Amounts of Recognized Assets (Liabilities)	(383)	(718)
Gross Amounts Offset in the Consolidated Statements of Financial Position	294	679
Net Amounts of Assets (Liabilities) Presented in the Consolidated Statements of Financial Position	(89)	(39)
Electricity
Offsetting Assets [Line Items]
Derivative Assets	307	342
Gross Amounts Offset in the Consolidated Statements of Financial Position	(232)	(280)
Net Amounts of Assets (Liabilities) Presented in the Consolidated Statements of Financial Position	75	62
Gross Amounts of Recognized Assets (Liabilities)	(305)	(342)
Gross Amounts Offset in the Consolidated Statements of Financial Position	253	298
Net Amounts of Assets (Liabilities) Presented in the Consolidated Statements of Financial Position	(52)	(44)
Other
Offsetting Assets [Line Items]
Derivative Assets	5	45
Gross Amounts Offset in the Consolidated Statements of Financial Position	(2)	(42)
Net Amounts of Assets (Liabilities) Presented in the Consolidated Statements of Financial Position	3	3
Gross Amounts of Recognized Assets (Liabilities)	(10)	(45)
Gross Amounts Offset in the Consolidated Statements of Financial Position	8	45
Net Amounts of Assets (Liabilities) Presented in the Consolidated Statements of Financial Position	(2)	0
Other derivative contracts
Offsetting Assets [Line Items]
Derivative Assets	12	4
Gross Amounts Offset in the Consolidated Statements of Financial Position	(9)	(3)
Net Amounts of Assets (Liabilities) Presented in the Consolidated Statements of Financial Position	3	1
Gross Amounts of Recognized Assets (Liabilities)	(7)	(5)
Gross Amounts Offset in the Consolidated Statements of Financial Position	7	3
Net Amounts of Assets (Liabilities) Presented in the Consolidated Statements of Financial Position	$ 0	$ (2)