Financial and Other Derivative Instruments (Netting Offsets of Derivative Assets and Liabilities) (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Derivative assets
Gross Amounts of Recognized Assets (Liabilities)	$ 2,340	$ 567
Gross Amounts Offset in the Consolidated Statements of Financial Position	(1,912)	(411)
Derivative assets, net	428	156
Derivative liabilities
Gross Amounts of Recognized Assets (Liabilities)	(2,557)	(533)
Gross Amounts Offset in the Consolidated Statements of Financial Position	1,848	405
Derivative liabilities, net	(709)	(128)
Natural gas
Derivative assets
Gross Amounts of Recognized Assets (Liabilities)	778	233
Gross Amounts Offset in the Consolidated Statements of Financial Position	(715)	(156)
Derivative assets, net	63	77
Derivative liabilities
Gross Amounts of Recognized Assets (Liabilities)	(1,055)	(223)
Gross Amounts Offset in the Consolidated Statements of Financial Position	664	151
Derivative liabilities, net	(391)	(72)
Electricity
Derivative assets
Gross Amounts of Recognized Assets (Liabilities)	952	180
Gross Amounts Offset in the Consolidated Statements of Financial Position	(627)	(120)
Derivative assets, net	325	60
Derivative liabilities
Gross Amounts of Recognized Assets (Liabilities)	(916)	(168)
Gross Amounts Offset in the Consolidated Statements of Financial Position	614	125
Derivative liabilities, net	(302)	(43)
Environmental & Other
Derivative assets
Gross Amounts of Recognized Assets (Liabilities)	610	154
Gross Amounts Offset in the Consolidated Statements of Financial Position	(570)	(135)
Derivative assets, net	40	19
Derivative liabilities
Gross Amounts of Recognized Assets (Liabilities)	(581)	(137)
Gross Amounts Offset in the Consolidated Statements of Financial Position	570	129
Derivative liabilities, net	(11)	(8)
Foreign currency exchange contracts
Derivative liabilities
Gross Amounts of Recognized Assets (Liabilities)	(5)	(5)
Gross Amounts Offset in the Consolidated Statements of Financial Position	0	0
Derivative liabilities, net	$ (5)	$ (5)