Employee Benefit Plans (Accumulated Other Comprehensive Loss Pretax Amounts Not Yet Reflected in Net Periodic Benefit Cost) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Pensions
Defined Benefit Plan Disclosure [Line Items]
Accumulated net actuarial losses	$ 835	$ 918
Accumulated prior service cost (credit)	0	1
Total	835	919
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Accumulated net actuarial losses	180	226
Accumulated prior service cost (credit)	(239)	(298)
Total	$ (59)	$ (72)