Pension and Other Postretirement Benefits Amounts reclassified from accumulated other comprehensive income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Income before income taxes	$ 201	$ 201	$ 353	$ 380
Income Tax Expense (Benefit)	(55)	(53)	(94)	(99)
Net income	147	148	260	281
Reclassification out of Accumulated Other Comprehensive Income | Defined benefit plans
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Actuarial losses	24	25	48	49
Prior service credit	(13)	(13)	(25)	(26)
Income before income taxes	11	12	23	23
Income Tax Expense (Benefit)	0	(4)	(1)	(6)
Net income	$ 11	$ 8	$ 22	$ 17