Retirement Benefit and Other Post-retirement Obligations - Summary of Effect of Members Living One Year More or One Year Less on the Defined Benefit Obligation (Detail) - Actuarial Assumption of Longevity [member] £ in Millions	Dec. 31, 2017 GBP (£)
UK Group plan [member]
Disclosure of sensitivity analysis for actuarial assumptions [line items]
(Decrease) /increase in defined benefit obligation, 1 year increase	£ 145
(Decrease) /increase in defined benefit obligation, 1 year decrease	(152)
US defined benefit pension plans [member]
Disclosure of sensitivity analysis for actuarial assumptions [line items]
(Decrease) /increase in defined benefit obligation, 1 year increase	8
(Decrease) /increase in defined benefit obligation, 1 year decrease	£ (9)