Pension and Other Postretirement Benefits Amounts reclassified from accumulated other comprehensive income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Income before income taxes	$ 293	$ 287	$ 646	$ 667
Income Tax Expense (Benefit)	(102)	(79)	(196)	(178)
Net income	193	208	453	489
Reclassification out of Accumulated Other Comprehensive Income | Defined benefit plans
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Actuarial losses	28	21	76	70
Prior service credit	(11)	(14)	(36)	(40)
Income before income taxes	17	7	40	30
Income Tax Expense (Benefit)	(6)	(4)	(7)	(10)
Net income	$ 11	$ 3	$ 33	$ 20