Comprehensive Loss (Reclassification Out of AOCI) (Details) - Reclassification out of Accumulated Other Comprehensive Income [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits [Member] | Amortization Of Defined Benefit Cost Items [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, pretax	$ (113)	$ (40)	$ (29)
Tax impact	43	15	11
Reclassifications, net of tax	(70)	(25)	(18)
Pension Benefits [Member] | Actuarial Gains (Losses) [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, pretax	(30)	(40)	(29)
Pension Benefits [Member] | Pension Settlement Cost [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, pretax	(83)
Reclassifications, net of tax	(51)
Postretirement Costs [Member] | Amortization Of Defined Benefit Cost Items [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, pretax	9	8	(3)
Tax impact	(3)	(3)	1
Reclassifications, net of tax	6	5	(2)
Postretirement Costs [Member] | Prior-Service Credits (Costs) [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, pretax	$ 9	9	5
Postretirement Costs [Member] | Actuarial Gains (Losses) [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, pretax		$ (1)	$ (8)