Pensions and Other Postretirement Benefits (Net Periodic Benefit Costs) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Defined Benefit Plan Disclosure [Line Items]
Pension settlement charge	$ 0	$ 0	$ 0	$ 22
Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost	8	8	16	17
Interest cost	24	25	48	49
Expected return on plan assets	(38)	(36)	(76)	(70)
Defined Benefit Plan, Amortization of Gain (Loss)	(16)	(19)	(32)	(38)
Pension settlement charge	0	0	0	22
Net periodic benefit cost	10	16	20	56
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	3	3	5	5
Interest cost	6	5	12	12
Defined Benefit Plan, Amortization of Gain (Loss)	(4)	(1)	(9)	(6)
Amortization of prior service credit	(15)	(17)	(30)	(30)
Net periodic benefit cost	$ (2)	$ (8)	$ (4)	$ (7)