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Pension and Postretirement Benefit Plans - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2020
Benefit Plan Information
Defined benefit pension and postretirement plans adjustment	$ (119)	$ (84)
Qualified and Non-qualified Pension Benefits \| Funded
Benefit Plan Information
Company contributions year to date	38
Postretirement Benefits \| Funded
Benefit Plan Information
Company contributions year to date	1
Total Accumulated Other Comprehensive Income (Loss)
Benefit Plan Information
Defined benefit pension and postretirement plans adjustment	$ (119)	$ (84)
Forecast \| Qualified and Non-qualified Pension Benefits \| Funded \| Maximum
Benefit Plan Information
Estimated pension and postretirement employer contributions in current fiscal year			$ 200