Pension and Other Postretirement Benefits - Schedule of Pension Plan Benefit Obligations (Details) - Pension benefits paid - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	$ 6,476	$ 7,359	$ 7,026
Accumulated benefit obligation	6,444	7,169
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	5,435	6,600
Fair value of plan assets	3,182	4,016
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	5,415	6,422
Fair value of plan assets	$ 3,179	$ 3,998