Employee Benefits (Details) £ in Millions, $ in Millions	3 Months Ended			6 Months Ended			12 Months Ended
	Jun. 30, 2018 GBP (£)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 GBP (£)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2017	Sep. 30, 2018 USD ($)
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		29	30		58	60
Expected return on plan assets, net of administration expenses		(50)	(49)		(101)	(97)
Amortization of prior-service cost		0	0		0	0
Amortization of net actuarial loss		7	8		15	15
Net periodic cost (benefit)		(14)	(11)		(28)	(22)
Loss on pension settlement		16	0		23	0
Total net periodic cost (benefit)		2	(11)		(5)	(22)
Payment for settlement				£ 99	132
Benefit obligation period increase (decrease)	£ (87)	(115)
Benefit obligation, (increase) decrease for settlement	£ 12	16		£ 17	23
Estimate of contributions to defined benefit pension plans for the current fiscal year		92			92
Contributions made to defined benefit pension plans		25	26		48	42
U.S.
Defined Benefit Plan Disclosure
Service cost		0	0		0	0
Interest cost		25	24		50	48
Expected return on plan assets, net of administration expenses		(36)	(35)		(72)	(70)
Amortization of prior-service cost		1	1		1	1
Amortization of net actuarial loss		15	12		30	25
Net periodic cost (benefit)		5	2		9	4
Loss on pension settlement		0	0		0	0
Total net periodic cost (benefit)		5	2		9	4
Estimate of contributions to defined benefit pension plans for the current fiscal year		143			143
Contributions made to defined benefit pension plans		8	13		25	26
Other
Defined Benefit Plan Disclosure
Service cost		0	0		0	0
Interest cost		7	6		14	12
Expected return on plan assets, net of administration expenses		(11)	(11)		(23)	(22)
Amortization of prior-service cost		0	0		0	0
Amortization of net actuarial loss		3	3		6	6
Net periodic cost (benefit)		(1)	(2)		(3)	(4)
Loss on pension settlement		0	0		0	0
Total net periodic cost (benefit)		(1)	(2)		(3)	(4)
Estimate of contributions to defined benefit pension plans for the current fiscal year		22			22
Contributions made to defined benefit pension plans		$ 3	$ 9		$ 11	$ 11
Scenario, Forecast | U.S.
Defined Benefit Plan Disclosure
Estimate of contributions to defined benefit pension plans for the current fiscal year								$ 100