Retirement Benefits - Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Pension Plan [Member] | UNITED STATES [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 16	$ 17	$ 32	$ 33
Interest cost	39	35	79	70
Expected return on plan assets	(63)	(69)	(127)	(137)
Settlement	0	0	0	0
Amortization of net loss	5	3	10	6
Amortization of prior service credit	0	0	0	0
Net periodic benefit (income)/cost	(3)	(14)	(6)	(28)
Pension Plan [Member] | UNITED KINGDOM [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	3	5	7	10
Interest cost	23	24	47	48
Expected return on plan assets	(62)	(77)	(125)	(155)
Settlement	0	20	0	20
Amortization of net loss	5	12	10	24
Amortization of prior service credit	(4)	(5)	(8)	(10)
Net periodic benefit (income)/cost	(35)	(21)	(69)	(63)
Pension Plan [Member] | Other Foreign Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	5	5	10	10
Interest cost	5	4	9	9
Expected return on plan assets	(8)	(7)	(15)	(15)
Settlement	0	0	0	0
Amortization of net loss	0	1	1	1
Amortization of prior service credit	0	0	0	0
Net periodic benefit (income)/cost	2	3	5	5
PRW [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0	0	0
Interest cost	1	1	2	2
Expected return on plan assets	0	0	0	0
Settlement	0	0	0	0
Amortization of net loss	0	0	0	0
Amortization of prior service credit	(1)	0	(2)	0
Net periodic benefit (income)/cost	$ 0	$ 1	$ 0	$ 2