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Employee Benefit Plans - Defined Benefit Pension Plans with an Accumulated Benefit Obligations in Excess of Plan Assets (Details) - USD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Retirement Benefits [Abstract]    
Projected benefit obligation $ 4,875 $ 4,498
Accumulated benefit obligation 4,875 4,498
Fair value of plan assets $ 4,363 $ 3,914