Employee Benefits - Pension Plans Narrative (Details) £ in Millions, $ in Millions	1 Months Ended		3 Months Ended					12 Months Ended
	Jul. 31, 2017 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 GBP (£)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Jun. 30, 2016 GBP (£)	Jun. 30, 2016 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				92.00%					92.00%
U.S.
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
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 the fair value of plan assets, PBO				$ 3,200	$ 2,900				$ 3,200	$ 2,900
Plans with ABO in excess of the fair value of plan assets, fair value of plan assets				2,000	1,700				2,000	1,700
Plans with ABO in excess of the fair value of plan assets, ABO				3,200	2,900				3,200	2,900
Benefit payments					281				152	139
Settlements									0	281
Defined benefit plan, benefit obligation, period increase (decrease)					325
Defined benefit plan, settlements, benefit obligation					158				0	281
Future amortization of loss				61					61
UK | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Future amortization of loss				42					42
U.K. | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Plans with PBO in excess of the fair value of plan assets, PBO				52	1,200				52	1,200
Plans with PBO in excess of the fair value of plan assets				30	1,100				30	1,100
Plans with ABO in excess of the fair value of plan assets, fair value of plan assets				30	1,100				30	1,100
Plans with ABO in excess of the fair value of plan assets, ABO				52	1,200				52	1,200
Benefit payments						£ 116	$ 159		146	242
Settlements								£ 371	496	159
Defined benefit plan, benefit obligation, period increase (decrease)			£ 325	434		103	141
Defined benefit plan, settlements, benefit obligation			£ 93	125		£ 42	$ 61		496	159
Effect of UK Plan insurance contract		$ 267
Other | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Plans with PBO in excess of the fair value of plan assets, PBO				1,400	1,200				1,400	1,200
Plans with PBO in excess of the fair value of plan assets				1,200	1,000				1,200	1,000
Plans with ABO in excess of the fair value of plan assets, fair value of plan assets				1,200	1,000				1,200	1,000
Plans with ABO in excess of the fair value of plan assets, ABO				$ 1,300	$ 1,100				1,300	1,100
Benefit payments									39	39
Settlements									0	0
Defined benefit plan, settlements, benefit obligation									$ 0	$ 0