Retirement Benefits (Components of the Net Periodic Benefit Cost) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost		$ 76	$ 178	$ 196
Interest cost		497	537	587
Expected return on plan assets		(921)	(940)	(977)
Amortization of prior service (credit) cost		(2)	(1)	(1)
Recognized actuarial loss (gain)		167	168	271
Net periodic benefit (credit) cost		(183)	(58)	76
Curtailment (gain) loss		(1)	(4)	5
Plan termination		0	0	0
Settlement loss		54	0	1
Total (credit) cost		(130)	(62)	82
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost		1	2	3
Interest cost		4	5	5
Expected return on plan assets		0	0	0
Amortization of prior service (credit) cost		1	4	3
Recognized actuarial loss (gain)		0	(2)	(1)
Net periodic benefit (credit) cost		6	9	10
Curtailment (gain) loss		0	0	0
Plan termination		0	0	(128)
Settlement loss		0	0	0
Total (credit) cost	$ (125)	6	9	(118)
Domestic Plan | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost		0	106	114
Interest cost		264	264	254
Expected return on plan assets		(357)	(379)	(373)
Amortization of prior service (credit) cost		0	0	0
Recognized actuarial loss (gain)		37	74	146
Net periodic benefit (credit) cost		(56)	65	141
Plan termination		0	0	0
Total (credit) cost		(56)	65	141
Domestic Plan | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost		0	0	1
Interest cost		2	2	2
Expected return on plan assets		0	0	0
Amortization of prior service (credit) cost		3	4	3
Recognized actuarial loss (gain)		(1)	(2)	(2)
Net periodic benefit (credit) cost		4	4	4
Plan termination		0	0	(128)
Total (credit) cost		4	4	(124)
Foreign Plan | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost		76	72	82
Interest cost		233	273	333
Expected return on plan assets		(564)	(561)	(604)
Amortization of prior service (credit) cost		(2)	(1)	(1)
Recognized actuarial loss (gain)		130	94	125
Net periodic benefit (credit) cost		(127)	(123)	(65)
Curtailment (gain) loss		(1)	(4)	5
Settlement loss		54	0	1
Total (credit) cost		(74)	(127)	(59)
Foreign Plan | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost		1	2	2
Interest cost		2	3	3
Expected return on plan assets		0	0	0
Amortization of prior service (credit) cost		(2)	0	0
Recognized actuarial loss (gain)		1	0	1
Net periodic benefit (credit) cost		2	5	6
Curtailment (gain) loss		0	0	0
Settlement loss		0	0	0
Total (credit) cost		$ 2	$ 5	$ 6