Financial and Other Derivative Instruments (Netting Offsets of Derivative Assets and Liabilities) (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Derivative assets
Derivative assets	$ 439	$ 549
Gross Amounts Offset in the Consolidated Statements of Financial Position	(301)	(468)
Derivative assets, net	138	81
Derivative liabilities
Derivative liabilities	(409)	(656)
Gross Amounts Offset in the Consolidated Statements of Financial Position	319	489
Derivative liabilities, net	(90)	(167)
Natural Gas
Derivative assets
Derivative assets	230	348
Gross Amounts Offset in the Consolidated Statements of Financial Position	(151)	(306)
Derivative assets, net	79	42
Derivative liabilities
Derivative liabilities	(204)	(461)
Gross Amounts Offset in the Consolidated Statements of Financial Position	151	321
Derivative liabilities, net	(53)	(140)
Electricity
Derivative assets
Derivative assets	202	193
Gross Amounts Offset in the Consolidated Statements of Financial Position	(148)	(157)
Derivative assets, net	54	36
Derivative liabilities
Derivative liabilities	(199)	(189)
Gross Amounts Offset in the Consolidated Statements of Financial Position	164	163
Derivative liabilities, net	(35)	(26)
Other
Derivative assets
Derivative assets	4	2
Gross Amounts Offset in the Consolidated Statements of Financial Position	0	0
Derivative assets, net	4	2
Derivative liabilities
Derivative liabilities	(2)	(3)
Gross Amounts Offset in the Consolidated Statements of Financial Position	2	2
Derivative liabilities, net	0	(1)
Foreign currency exchange contracts
Derivative assets
Derivative assets	3	6
Gross Amounts Offset in the Consolidated Statements of Financial Position	(2)	(5)
Derivative assets, net	1	1
Derivative liabilities
Derivative liabilities	(4)	(3)
Gross Amounts Offset in the Consolidated Statements of Financial Position	2	3
Derivative liabilities, net	$ (2)	$ 0