Plant Closures (Details) - USD ($)	3 Months Ended	24 Months Ended
Dec. 31, 2021	Dec. 31, 2020	Mar. 31, 2021
Property, Plant and Equipment [Line Items]
Accelerated depreciation	$ 35,000,000	$ 33,000,000	$ 377,000,000
Depreciation expense	453,000,000	$ 345,000,000
Additional depreciation for coal-fired fleet
Property, Plant and Equipment [Line Items]
Depreciation expense	$ 82,000,000

The amount of expense recognized in the current period that reflects the allocation of the cost of tangible assets over the assets' useful lives. Includes production and non-production related depreciation.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=123570139&loc=d3e3602-108585

Name:	us-gaap_Depreciation
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration