Financial and Other Derivative Instruments (Netting Offsets of Derivative Assets and Liabilities) (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Derivative assets
Gross Amounts of Recognized Assets (Liabilities)	$ 1,041	$ 567
Gross Amounts Offset in the Consolidated Statements of Financial Position	(820)	(411)
Derivative assets, net	221	156
Derivative liabilities
Gross Amounts of Recognized Assets (Liabilities)	(1,216)	(533)
Gross Amounts Offset in the Consolidated Statements of Financial Position	820	405
Derivative liabilities, net	(396)	(128)
Natural gas
Derivative assets
Gross Amounts of Recognized Assets (Liabilities)	336	233
Gross Amounts Offset in the Consolidated Statements of Financial Position	(289)	(156)
Derivative assets, net	47	77
Derivative liabilities
Gross Amounts of Recognized Assets (Liabilities)	(528)	(223)
Gross Amounts Offset in the Consolidated Statements of Financial Position	289	151
Derivative liabilities, net	(239)	(72)
Electricity
Derivative assets
Gross Amounts of Recognized Assets (Liabilities)	330	180
Gross Amounts Offset in the Consolidated Statements of Financial Position	(192)	(120)
Derivative assets, net	138	60
Derivative liabilities
Gross Amounts of Recognized Assets (Liabilities)	(325)	(168)
Gross Amounts Offset in the Consolidated Statements of Financial Position	192	125
Derivative liabilities, net	(133)	(43)
Environmental & Other
Derivative assets
Gross Amounts of Recognized Assets (Liabilities)	375	154
Gross Amounts Offset in the Consolidated Statements of Financial Position	(339)	(135)
Derivative assets, net	36	19
Derivative liabilities
Gross Amounts of Recognized Assets (Liabilities)	(356)	(137)
Gross Amounts Offset in the Consolidated Statements of Financial Position	339	129
Derivative liabilities, net	$ (17)	$ (8)