Benefit Plans and Other Postretirement Benefits - Net Benefit Costs and Funded Status (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Sep. 30, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Annual periodic pension cost
Curtailment gain	$ (8)
Pension Benefits
Annual periodic pension cost
Service cost benefits earned		$ 10	$ 10	$ 23
Interest cost on benefit obligation		38	46	44
Expected return on plan assets		(61)	(59)	(62)
Amortization of unrecognized net loss		(5)	(3)	0
Curtailment gain		0	0	(7)
Net periodic benefit (credit)/cost		(8)	0	12
Pension and other post retirement benefit obligations
Benefit obligation at January 1		1,397	1,222
Service cost		10	10	23
Interest cost		38	46	44
Plan amendments		0	0
Actuarial loss		126	207
Employee and retiree contributions		0	0
Benefit payments		(82)	(88)
Benefit obligation at December 31		1,489	1,397	1,222
Fair value of plan assets for pension and other post retirement benefit
Fair value of plan assets at January 1		1,150	981
Actual return on plan assets		193	216
Employee and retiree contributions		0	0
Employer contributions		11	41
Benefit payments		(82)	(88)
Fair value of plan assets at December 31		1,272	1,150	981
Funded status at December 31 — excess of obligation over assets		(217)	(247)
Other Postretirement Benefits Plan
Annual periodic pension cost
Service cost benefits earned		0	1	1
Interest cost on benefit obligation		3	3	4
Amortization of unrecognized prior service cost		(14)	(13)	(10)
Amortization of unrecognized net loss		(1)	0	0
Curtailment gain		0	0	(10)
Net periodic benefit (credit)/cost		(10)	(9)	(15)
Pension and other post retirement benefit obligations
Benefit obligation at January 1		93	83
Service cost		0	1	1
Interest cost		3	3	4
Plan amendments		0	(2)
Actuarial loss		0	16
Employee and retiree contributions		3	4
Benefit payments		(9)	(12)
Benefit obligation at December 31		90	93	83
Fair value of plan assets for pension and other post retirement benefit
Fair value of plan assets at January 1		0	0
Actual return on plan assets		0	0
Employee and retiree contributions		3	4
Employer contributions		6	7
Benefit payments		(9)	(11)
Fair value of plan assets at December 31		0	0	$ 0
Funded status at December 31 — excess of obligation over assets		$ (90)	$ (93)

Amount of increase (decrease) in plan assets of defined benefit plan from actual return (loss) determined by change in fair value of plan assets adjusted for contributions, benefit payments, and other expenses.

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

- Definition

Amount of gain (loss) from change in actuarial assumptions which (increases) decreases benefit obligation of defined benefit plan. Assumptions include, but are not limited to, interest, mortality, employee turnover, salary, and temporary deviation from substantive plan.