Pension and Other Postretirement Benefits - Schedule of Pension Plan Benefit Obligations (Details) - Pension benefits paid - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	$ 2,713	$ 7,249	$ 6,476
Accumulated benefit obligation	2,707	7,219
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	2,364	6,064
Fair value of plan assets	1,364	3,579
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	$ 2,359	6,045
Fair value of plan assets		$ 3,579