Financial and Other Derivative Instruments (Netting Offsets of Derivative Assets and Liabilities) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Derivative assets
Derivative assets	$ 397	$ 652
Gross Amounts Offset in the Consolidated Statements of Financial Position	(255)	(498)
Derivative assets, net	142	154
Derivative liabilities
Derivative liabilities	(397)	(657)
Gross Amounts Offset in the Consolidated Statements of Financial Position	270	511
Derivative liabilities, net	(127)	(146)
Natural gas
Derivative assets
Derivative assets	207	357
Gross Amounts Offset in the Consolidated Statements of Financial Position	(118)	(256)
Derivative assets, net	89	101
Derivative liabilities
Derivative liabilities	(196)	(378)
Gross Amounts Offset in the Consolidated Statements of Financial Position	126	263
Derivative liabilities, net	(70)	(115)
Electricity
Derivative assets
Derivative assets	183	285
Gross Amounts Offset in the Consolidated Statements of Financial Position	(135)	(241)
Derivative assets, net	48	44
Derivative liabilities
Derivative liabilities	(198)	(275)
Gross Amounts Offset in the Consolidated Statements of Financial Position	142	246
Derivative liabilities, net	(56)	(29)
Other
Derivative assets
Derivative assets	5	9
Gross Amounts Offset in the Consolidated Statements of Financial Position	0	0
Derivative assets, net	5	9
Derivative liabilities
Derivative liabilities	(1)	(1)
Gross Amounts Offset in the Consolidated Statements of Financial Position	0	1
Derivative liabilities, net	(1)	0
Foreign currency exchange contracts
Derivative assets
Derivative assets	2	1
Gross Amounts Offset in the Consolidated Statements of Financial Position	(2)	(1)
Derivative assets, net	0	0
Derivative liabilities
Derivative liabilities	(2)	(3)
Gross Amounts Offset in the Consolidated Statements of Financial Position	2	1
Derivative liabilities, net	$ 0	$ (2)