Other Postretirement Employee Benefit Plans (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2018
Other postretirement benefits
Defined Benefit Plan Disclosure [Line Items]
Postretirement liability, noncurrent	$ (152)		$ (131)
Medical trend rate - initial	6.30%		6.00%
Medical trend rate - ultimate	4.50%		4.50%
Unrecognized prior service credits, before tax			$ 19
Unrecognized net prior service credits, net of tax			14
Unrecognized actuarial losses, before tax			(6)
Unrecognized actuarial losses, net of tax			(5)
Future amortization of prior service cost (credit)			(2)
Future amortization of prior service cost (credit), net of tax			$ (2)
ASU No. 2017-07
Defined Benefit Plan Disclosure [Line Items]
Effect of change in accounting principle on operating profit	$ (31)	$ (16)
Effect of change in accounting principle on net other income	31	16
ASU No. 2017-07 | Other postretirement benefits
Defined Benefit Plan Disclosure [Line Items]
Effect of change in accounting principle on operating profit	2	3
Effect of change in accounting principle on net other income	$ (2)	$ (3)