Retirement Benefits - Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Pension Plan [Member] | UNITED STATES [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 17	$ 16	$ 50	$ 49
Interest cost	35	34	105	104
Expected return on plan assets	(68)	(61)	(205)	(184)
Settlement	0	1	0	1
Amortization of net loss	3	4	9	10
Amortization of prior service credit	0	0	0	0
Net periodic benefit (income)/cost	(13)	(6)	(41)	(20)
Pension Plan [Member] | UNITED KINGDOM [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	4	8	14	23
Interest cost	24	23	72	69
Expected return on plan assets	(72)	(72)	(227)	(211)
Settlement	12	0	32	0
Amortization of net loss	10	13	34	39
Amortization of prior service credit	(4)	(4)	(14)	(13)
Net periodic benefit (income)/cost	(26)	(32)	(89)	(93)
Pension Plan [Member] | Other Foreign Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	6	6	16	15
Interest cost	4	4	13	13
Expected return on plan assets	(8)	(7)	(23)	(22)
Settlement	(14)	0	(14)	0
Amortization of net loss	0	0	1	1
Amortization of prior service credit	0	0	0	0
Net periodic benefit (income)/cost	(12)	3	(7)	7
PRW [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0	0	0
Interest cost	1	1	3	3
Expected return on plan assets	0	0	0	0
Settlement	0	0	0	0
Amortization of net loss	1	0	1	0
Amortization of prior service credit	0	0	0	0
Net periodic benefit (income)/cost	$ 2	$ 1	$ 4	$ 3