Employee Benefits (Details) £ in Millions, $ in Millions	3 Months Ended			9 Months Ended			12 Months Ended
	Sep. 30, 2018 GBP (£)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 GBP (£)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2017
Defined Benefit Plan Disclosure
Federal statutory income tax rate							35.00%
U.K.
Defined Benefit Plan Disclosure
Service cost		$ 0	$ 0		$ 0	$ 0
Interest cost		26	31		84	91
Expected return on plan assets, net of administration expenses		(46)	(50)		(147)	(147)
Amortization of prior-service cost		1	0		1	0
Amortization of net actuarial loss		6	8		21	23
Net periodic (benefit) cost		(13)	(11)		(41)	(33)
Loss on pension settlement		9	0		32	0
Total net periodic (benefit) cost		(4)	(11)		(9)	(33)
Payment for settlement				£ 125	164
Benefit obligation period increase (decrease)	£ (108)	(143)
Benefit obligation, (increase) decrease for settlement	£ 7	9		£ 24	32
Expected employer contributions current fiscal year		92			92
Contributions made to defined benefit pension plans		27	22		75	64
U.S.
Defined Benefit Plan Disclosure
Service cost		0	0		0	0
Interest cost		24	24		74	72
Expected return on plan assets, net of administration expenses		(36)	(34)		(108)	(104)
Amortization of prior-service cost		0	0		1	1
Amortization of net actuarial loss		15	13		45	38
Net periodic (benefit) cost		3	3		12	7
Loss on pension settlement		0	0		0	0
Total net periodic (benefit) cost		3	3		12	7
Expected employer contributions current fiscal year		143			143
Contributions to defined benefit pension plans for the current fiscal year at 35% Federal tax rate		100
Contributions made to defined benefit pension plans		108	5		133	31
Other
Defined Benefit Plan Disclosure
Service cost		0	0		0	0
Interest cost		6	7		20	19
Expected return on plan assets, net of administration expenses		(11)	(13)		(34)	(35)
Amortization of prior-service cost		0	0		0	0
Amortization of net actuarial loss		3	3		9	9
Net periodic (benefit) cost		(2)	(3)		(5)	(7)
Loss on pension settlement		0	0		0	0
Total net periodic (benefit) cost		(2)	(3)		(5)	(7)
Expected employer contributions current fiscal year		22			22
Contributions made to defined benefit pension plans		$ 3	$ 3		$ 14	$ 14