Pension and Other Postretirement Benefits - Amounts reclassified from accumulated other comprehensive income (Details) - USD ($) $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Income before income taxes	$ 335			$ 257			$ 678	$ 695
Income Tax Expense (Benefit)	(91)			(54)			(182)	(190)
Net income	245	$ 141	$ 114	204	$ 174	$ 131	500	509
Reclassification out of Accumulated Other Comprehensive Income | Defined benefit plans
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Actuarial losses	20			21			63	70
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Settlement	5			6			61	37
Prior service credit	(5)			(7)			(18)	(25)
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Curtailment	0			0			0	(14)
Income before income taxes	20			20			106	68
Income Tax Expense (Benefit)	(5)			(5)			(21)	(13)
Net income	$ 15			$ 15			$ 85	$ 55