Employee Benefit Plans - Schedule of Allocation of Plan Assets (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Amount	$ 3,000	$ 2,825
Percent of Total Assets	100.00%	100.00%
Growth-Oriented
Defined Benefit Plan Disclosure [Line Items]
Amount	$ 1,816	$ 2,027
Percent of Total Assets	61.00%	72.00%
Equity
Defined Benefit Plan Disclosure [Line Items]
Amount	$ 1,324	$ 1,772
Percent of Total Assets	44.00%	63.00%
Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Amount	$ 271	$ 229
Percent of Total Assets	9.00%	8.00%
Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Amount	$ 221	$ 26
Percent of Total Assets	8.00%	1.00%
Immunizing
Defined Benefit Plan Disclosure [Line Items]
Amount	$ 1,184	$ 798
Percent of Total Assets	39.00%	28.00%
Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Amount	$ 1,018	$ 595
Percent of Total Assets	34.00%	21.00%
Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Amount	$ 166	$ 203
Percent of Total Assets	5.00%	7.00%