Benefit Plans and Other Postretirement Benefits - Net Benefit Costs and Funded Status (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Annual periodic pension cost
Curtailment loss			$ (8)
Pension and other post retirement benefit obligations
Curtailment loss			(32)
Pension Benefits
Annual periodic pension cost
Service cost benefits earned	$ 9	$ 10	10
Interest cost on benefit obligation	27	38	46
Expected return on plan assets	(66)	(61)	(59)
Amortization of unrecognized net loss	1	5	3
Curtailment loss	2	0	0
Net periodic benefit (credit)/cost	(27)	(8)	0
Pension and other post retirement benefit obligations
Benefit obligation at January 1	1,489	1,397
Acquired benefit obligation from Direct Energy	74	0
Service cost	9	10	10
Interest cost	27	38	46
Actuarial (gain)/loss	(55)	126
Employee and retiree contributions	0	0
Curtailment loss	0	0
Benefit payments	(93)	(82)
Foreign exchange translation	1	0
Benefit obligation at December 31	1,452	1,489	1,397
Fair value of plan assets for pension and other post retirement benefit
Fair value of plan assets at January 1	1,272	1,150
Acquired fair value of plan assets from Direct Energy	64	0
Actual return on plan assets	85	193
Employee and retiree contributions	0	0
Employer contributions	7	11
Benefit payments	(93)	(82)
Foreign exchange translation	1	0
Fair value of plan assets at December 31	1,336	1,272	1,150
Funded status at December 31 — excess of obligation over assets	(116)	(217)
Other Postretirement Benefits Plan
Annual periodic pension cost
Service cost benefits earned	0	0	1
Interest cost on benefit obligation	2	3	3
Amortization of unrecognized prior service cost	(10)	(14)	(13)
Amortization of unrecognized net loss	1	1	0
Curtailment loss	1	0	0
Net periodic benefit (credit)/cost	(6)	(10)	(9)
Pension and other post retirement benefit obligations
Benefit obligation at January 1	90	93
Acquired benefit obligation from Direct Energy	19	0
Service cost	0	0	1
Interest cost	2	3	3
Actuarial (gain)/loss	0	0
Employee and retiree contributions	3	3
Curtailment loss	1	0
Benefit payments	(10)	(9)
Foreign exchange translation	0	0
Benefit obligation at December 31	105	90	93
Fair value of plan assets for pension and other post retirement benefit
Fair value of plan assets at January 1	0	0
Acquired fair value of plan assets from Direct Energy		0
Actual return on plan assets	0	0
Employee and retiree contributions	3	3
Employer contributions	7	6
Benefit payments	(10)	(9)
Foreign exchange translation	0
Fair value of plan assets at December 31	0	0	$ 0
Funded status at December 31 — excess of obligation over assets	$ (105)	$ (90)