Financial and Other Derivative Instruments (Netting Offsets of Derivative Assets and Liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Offsetting Assets [Line Items]
Gross Amounts of Recognized Assets (Liabilities)	$ 652	$ 549
Gross Amounts Offset in the Consolidated Statements of Financial Position	(498)	(468)
Derivative assets, net	154	81
Gross Amounts of Recognized Assets (Liabilities)	(657)	(656)
Gross Amounts Offset in the Consolidated Statements of Financial Position	511	489
Derivative liabilities, net	(146)	(167)
Natural Gas
Offsetting Assets [Line Items]
Gross Amounts of Recognized Assets (Liabilities)	357	348
Gross Amounts Offset in the Consolidated Statements of Financial Position	(256)	(306)
Derivative assets, net	101	42
Gross Amounts of Recognized Assets (Liabilities)	(378)	(461)
Gross Amounts Offset in the Consolidated Statements of Financial Position	263	321
Derivative liabilities, net	(115)	(140)
Electricity
Offsetting Assets [Line Items]
Gross Amounts of Recognized Assets (Liabilities)	285	193
Gross Amounts Offset in the Consolidated Statements of Financial Position	(241)	(157)
Derivative assets, net	44	36
Gross Amounts of Recognized Assets (Liabilities)	(275)	(189)
Gross Amounts Offset in the Consolidated Statements of Financial Position	246	163
Derivative liabilities, net	(29)	(26)
Other
Offsetting Assets [Line Items]
Gross Amounts of Recognized Assets (Liabilities)	9	2
Gross Amounts Offset in the Consolidated Statements of Financial Position	0	0
Derivative assets, net	9	2
Gross Amounts of Recognized Assets (Liabilities)	(1)	(3)
Gross Amounts Offset in the Consolidated Statements of Financial Position	1	2
Derivative liabilities, net	0	(1)
Foreign currency exchange contracts
Offsetting Assets [Line Items]
Gross Amounts of Recognized Assets (Liabilities)	1	6
Gross Amounts Offset in the Consolidated Statements of Financial Position	(1)	(5)
Derivative assets, net	0	1
Gross Amounts of Recognized Assets (Liabilities)	(3)	(3)
Gross Amounts Offset in the Consolidated Statements of Financial Position	1	3
Derivative liabilities, net	$ (2)	$ 0