Pension and Other Postretirement Benefits - Schedule of Pension Plan Benefit Obligations (Details) - Pension benefits paid - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	$ 7,249	$ 6,476	$ 7,359
Accumulated benefit obligation	7,219	6,444
The aggregate projected benefit obligation and fair value of plan assets for pension plans with projected benefit obligations in excess of plan assets was as follows:
Projected benefit obligation	6,064	5,435
Fair value of plan assets	3,579	3,182
The aggregate accumulated benefit obligation and fair value of plan assets for pension plans with accumulated benefit obligations in excess of plan assets was as follows:
Accumulated benefit obligation	6,045	5,415
Fair value of plan assets	$ 3,579	$ 3,179