Retirement Benefits - Net Periodic Pension Costs (Benefit) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Amortization of:
Curtailment gain	$ 0	$ 0	$ (32)
Settlement loss	48	26	0
Postretirement Health Care Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0	1
Interest cost	3	4	8
Expected return on plan assets	(10)	(9)	(9)
Amortization of:
Unrecognized net loss	5	5	8
Unrecognized prior service benefit	(18)	(27)	(59)
Curtailment gain	0	0	0
Settlement loss	0	0	0
Net periodic cost (benefit)	(20)	(27)	(51)
U.S. | Pension Benefit Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0	0
Interest cost	185	182	193
Expected return on plan assets	(229)	(220)	(212)
Amortization of:
Unrecognized net loss	44	37	46
Unrecognized prior service benefit	0	0	0
Curtailment gain	0	0	0
Settlement loss	0	0	0
Net periodic cost (benefit)	0	(1)	27
Non U.S. | Pension Benefit Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	3	11	12
Interest cost	40	55	66
Expected return on plan assets	(92)	(93)	(103)
Amortization of:
Unrecognized net loss	16	11	17
Unrecognized prior service benefit	0	0	(3)
Curtailment gain	0	0	(32)
Settlement loss	48	26	0
Net periodic cost (benefit)	$ 15	$ 10	$ (43)