Financial and Other Derivative Instruments (Netting Offsets of Derivative Assets and Liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Offsetting Assets [Line Items]
Gross Amounts of Recognized Assets (Liabilities)	$ 549	$ 711
Gross Amounts Offset in the Consolidated Statements of Financial Position	(468)	(528)
Net Amounts of Assets (Liabilities) Presented in the Consolidated Statements of Financial Position	81	183
Gross Amounts of Recognized Assets (Liabilities)	(656)	(705)
Gross Amounts Offset in the Consolidated Statements of Financial Position	489	562
Net Amounts of Assets (Liabilities) Presented in the Consolidated Statements of Financial Position	(167)	(143)
Natural Gas
Offsetting Assets [Line Items]
Gross Amounts of Recognized Assets (Liabilities)	348	387
Gross Amounts Offset in the Consolidated Statements of Financial Position	(306)	(285)
Net Amounts of Assets (Liabilities) Presented in the Consolidated Statements of Financial Position	42	102
Gross Amounts of Recognized Assets (Liabilities)	(461)	(383)
Gross Amounts Offset in the Consolidated Statements of Financial Position	321	294
Net Amounts of Assets (Liabilities) Presented in the Consolidated Statements of Financial Position	(140)	(89)
Electricity
Offsetting Assets [Line Items]
Gross Amounts of Recognized Assets (Liabilities)	193	307
Gross Amounts Offset in the Consolidated Statements of Financial Position	(157)	(232)
Net Amounts of Assets (Liabilities) Presented in the Consolidated Statements of Financial Position	36	75
Gross Amounts of Recognized Assets (Liabilities)	(189)	(305)
Gross Amounts Offset in the Consolidated Statements of Financial Position	163	253
Net Amounts of Assets (Liabilities) Presented in the Consolidated Statements of Financial Position	(26)	(52)
Other
Offsetting Assets [Line Items]
Gross Amounts of Recognized Assets (Liabilities)	2	5
Gross Amounts Offset in the Consolidated Statements of Financial Position	0	(2)
Net Amounts of Assets (Liabilities) Presented in the Consolidated Statements of Financial Position	2	3
Gross Amounts of Recognized Assets (Liabilities)	(3)	(10)
Gross Amounts Offset in the Consolidated Statements of Financial Position	2	8
Net Amounts of Assets (Liabilities) Presented in the Consolidated Statements of Financial Position	(1)	(2)
Foreign currency exchange contracts
Offsetting Assets [Line Items]
Gross Amounts of Recognized Assets (Liabilities)	6	12
Gross Amounts Offset in the Consolidated Statements of Financial Position	(5)	(9)
Net Amounts of Assets (Liabilities) Presented in the Consolidated Statements of Financial Position	1	3
Gross Amounts of Recognized Assets (Liabilities)	(3)	(7)
Gross Amounts Offset in the Consolidated Statements of Financial Position	3	7
Net Amounts of Assets (Liabilities) Presented in the Consolidated Statements of Financial Position	$ 0	$ 0