Benefit Obligations (Schedule of Net Periodic Benefit Costs Recognized) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Recognized actuarial (gain) loss	$ 46	$ 103	$ 127
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	9	8	12
Interest cost	107	113	139
Expected return on plan assets	(198)	(177)	(209)
Amortization of prior service cost / (credit)	0	0	0
Recognized actuarial (gain) loss	48	101	134
Curtailment (gain) loss	0	0	(3)
Settlement (gain) loss	0	0	0
Special termination benefit	1	3	2
Total	(33)	48	75
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	1	0	1
Interest cost	1	2	3
Expected return on plan assets	0	0	0
Amortization of prior service cost / (credit)	(1)	(3)	0
Recognized actuarial (gain) loss	(2)	2	(7)
Curtailment (gain) loss	0	0	0
Settlement (gain) loss	0	0	0
Special termination benefit	0	0	0
Total	$ (1)	$ 1	$ (3)