Employee Benefit Plans - Schedule of Changes in Benefit Obligation and Fair Value of Plan Assets (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2020	Dec. 31, 2019
Change in Plan Assets:
Fair Value of Plan Assets at Beginning of Plan Year	$ 2,825,000,000
Non-qualified Employer Contributions	0	$ 0	$ 0
Fair Value of Plan Assets at End of Plan Year	3,000,000,000	2,825,000,000
Capitalized service costs	3,000,000	3,000,000
Pension Benefits
Actuarial Present Value of Benefit Obligation
Accumulated Benefit Obligation				$ 3,134,000,000	$ 2,963,000,000
Projected Benefit Obligation	3,122,000,000	3,122,000,000	2,758,000,000	3,257,000,000	3,122,000,000
Change in Projected Benefit Obligation:
Projected Benefit Obligation at Beginning of Plan Year	3,122,000,000	2,758,000,000
Service Cost	46,000,000	34,000,000
Interest Cost	82,000,000	103,000,000	92,000,000
Plan Participants' Contributions	0	0
Actuarial Loss (Gain)	197,000,000	418,000,000
Benefits Paid	(190,000,000)	(191,000,000)
Benefit Obligation at End of Plan Year	3,257,000,000	3,122,000,000	2,758,000,000
Change in Plan Assets:
Fair Value of Plan Assets at Beginning of Plan Year	2,825,000,000	2,431,000,000
Actual Return on Plan Assets	345,000,000	568,000,000
Plan Participants' Contributions	0	0
Benefits Paid	(190,000,000)	(191,000,000)
Fair Value of Plan Assets at End of Plan Year	3,000,000,000	2,825,000,000	2,431,000,000
Funded Status at End of Plan Year				(257,000,000)	(297,000,000)
Pension Benefits | Nonqualified Plan
Change in Plan Assets:
Non-qualified Employer Contributions	20,000,000	17,000,000
Post-retirement Benefits
Actuarial Present Value of Benefit Obligation
Projected Benefit Obligation	117,000,000	118,000,000	118,000,000	96,000,000	117,000,000
Change in Projected Benefit Obligation:
Projected Benefit Obligation at Beginning of Plan Year	117,000,000	118,000,000
Service Cost	1,000,000	1,000,000
Interest Cost	5,000,000	2,000,000	7,000,000
Plan Participants' Contributions	6,000,000	7,000,000
Actuarial Loss (Gain)	(11,000,000)	22,000,000
Benefits Paid	(22,000,000)	(33,000,000)
Benefit Obligation at End of Plan Year	96,000,000	117,000,000	118,000,000
Change in Plan Assets:
Fair Value of Plan Assets at Beginning of Plan Year	0	0
Actual Return on Plan Assets	0	0
Plan Participants' Contributions	6,000,000	7,000,000
Benefits Paid	(22,000,000)	(33,000,000)
Fair Value of Plan Assets at End of Plan Year	0	0	$ 0
Funded Status at End of Plan Year				$ (96,000,000)	$ (117,000,000)
Post-retirement Benefits | Nonqualified Plan
Change in Plan Assets:
Non-qualified Employer Contributions	$ 16,000,000	$ 26,000,000