Retirement Benefits - Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Pension Plan [Member] | United States [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 18	$ 16	$ 36	$ 32
Interest cost	33	39	66	79
Expected return on plan assets	(72)	(63)	(145)	(127)
Settlement	2	0	2	0
Amortization of net loss	8	5	17	10
Amortization of prior service credit	0	0	0	0
Net periodic benefit (income)/cost	(11)	(3)	(24)	(6)
Pension Plan [Member] | United Kingdom [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	3	3	7	7
Interest cost	18	23	36	47
Expected return on plan assets	(59)	(62)	(121)	(125)
Settlement	1	0	1	0
Amortization of net loss	5	5	11	10
Amortization of prior service credit	(4)	(4)	(8)	(8)
Net periodic benefit (income)/cost	(36)	(35)	(74)	(69)
Pension Plan [Member] | Other Foreign Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	5	5	10	10
Interest cost	3	5	7	9
Expected return on plan assets	(9)	(8)	(17)	(15)
Settlement	0	0	0	0
Amortization of net loss	2	0	2	1
Amortization of prior service credit	0	0	0	0
Net periodic benefit (income)/cost	1	2	2	5
PRW [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0	0	0
Interest cost	0	1	1	2
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	(1)	(1)	(2)	(2)
Net periodic benefit (income)/cost	$ 0	$ 0	$ 0	$ 0