Employee Benefits - Pension Plans Narrative (Details) £ in Millions, $ in Millions	1 Months Ended		3 Months Ended						12 Months Ended
	Sep. 30, 2018 USD ($)	Jul. 31, 2017 USD ($)	Dec. 31, 2018 GBP (£)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 GBP (£)	Dec. 31, 2017 USD ($)	Jun. 30, 2017 GBP (£)	Jun. 30, 2017 USD ($)	Dec. 31, 2018 GBP (£)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 GBP (£)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Percentage of the Company's projected benefit obligation				90.00%						90.00%
U.S.
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Voluntary contribution	$ 100
Non-cash contributions by employer		$ 80
U.S. | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Plans with PBO in excess of plan assets, PBO				$ 2,900		$ 3,200				$ 2,900		$ 3,200
Plans with ABO in excess of the fair value of plan assets, ABO				2,900		3,200				2,900		3,200
Plans with ABO in excess of the fair value of plan assets, fair value of plan assets				1,800		2,000				1,800		2,000
Settlements										0		0
Defined benefit plan, benefit obligation, period increase (decrease)						325
Defined benefit plan, settlements, benefit obligation						158				0		0
Settlement expense										0		0	$ 158
Benefit payments						281				156		152
Future amortization of loss				55						55
U.K. | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Plans with PBO in excess of plan assets, PBO				47		52				47		52
Plans with ABO in excess of the fair value of plan assets, ABO				47		52				47		52
Plans with ABO in excess of the fair value of plan assets, fair value of plan assets				30		30				30		30
Plans with PBO in excess of plan assets, plan assets				30		30				30		30
Settlements									£ 139	176	£ 371	496
Defined benefit plan, benefit obligation, period increase (decrease)			£ 122	154	£ 325	434	£ 103	$ 141
Defined benefit plan, settlements, benefit obligation							42	61		176		496
Settlement expense					£ (93)	125			£ (28)	37		125	61
Benefit payments							£ 116	$ 159		160		146
Other | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Plans with PBO in excess of plan assets, PBO				1,200		1,400				1,200		1,400
Plans with ABO in excess of the fair value of plan assets, ABO				363		1,300				363		1,300
Plans with ABO in excess of the fair value of plan assets, fair value of plan assets				271		1,200				271		1,200
Plans with PBO in excess of plan assets, plan assets				1,100		$ 1,200				1,100		1,200
Settlements										0		0
Defined benefit plan, settlements, benefit obligation										0		0
Settlement expense										0		0	$ 0
Benefit payments										43		$ 39
Foreign Plan | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Future amortization of loss				$ 42						$ 42