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Pension and Other Postretirement Benefits - Pension Plan Benefit Obligations (Details) - USD ($)
$ in Millions
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
The aggregate projected benefit obligation and fair value of plan assets for defined benefit pension plans with projected benefit obligations in excess of plan assets was as follows:      
Projected benefit obligation $ 2,472 $ 3,795  
Fair value of plan assets 1,747 2,444  
Pension benefits​      
The projected benefit obligation and accumulated benefit obligation for all defined benefit pension plans was as follows:      
Projected benefit obligation 2,817 4,081 $ 142
Accumulated benefit obligation 2,807 4,068  
The aggregate accumulated benefit obligation and fair value of plan assets for defined benefit pension plans with accumulated benefit obligations in excess of plan assets was as follows:      
Accumulated benefit obligation 2,464 3,784  
Fair value of plan assets $ 1,747 $ 2,444