LIABILITIES ARISING FROM INSURANCE CONTRACTS AND PARTICIPATING INVESTMENT CONTRACTS (Details) - Schedule of Insurance Contracts - GBP (£) £ in Millions		Dec. 31, 2017	Dec. 31, 2016
LIABILITIES ARISING FROM INSURANCE CONTRACTS AND PARTICIPATING INVESTMENT CONTRACTS (Details) - Schedule of Insurance Contracts [Line Items]
Insurance contracts		£ 103,413	£ 94,390
Gross amount arising from insurance contracts [member]
LIABILITIES ARISING FROM INSURANCE CONTRACTS AND PARTICIPATING INVESTMENT CONTRACTS (Details) - Schedule of Insurance Contracts [Line Items]
Insurance contracts		103,413	94,390
Reinsurer's share of amount arising from insurance contracts [member]
LIABILITIES ARISING FROM INSURANCE CONTRACTS AND PARTICIPATING INVESTMENT CONTRACTS (Details) - Schedule of Insurance Contracts [Line Items]
Insurance contracts	[1]	(576)	(685)
Net [Member]
LIABILITIES ARISING FROM INSURANCE CONTRACTS AND PARTICIPATING INVESTMENT CONTRACTS (Details) - Schedule of Insurance Contracts [Line Items]
Insurance contracts		102,837	93,705
Life insurance contracts [member] | Gross amount arising from insurance contracts [member]
LIABILITIES ARISING FROM INSURANCE CONTRACTS AND PARTICIPATING INVESTMENT CONTRACTS (Details) - Schedule of Insurance Contracts [Line Items]
Insurance contracts		102,830	93,777
Life insurance contracts [member] | Gross amount arising from insurance contracts [member] | Insurance contracts [member]
LIABILITIES ARISING FROM INSURANCE CONTRACTS AND PARTICIPATING INVESTMENT CONTRACTS (Details) - Schedule of Insurance Contracts [Line Items]
Insurance contracts		89,157	79,793
Life insurance contracts [member] | Gross amount arising from insurance contracts [member] | Participating investment contracts [Member]
LIABILITIES ARISING FROM INSURANCE CONTRACTS AND PARTICIPATING INVESTMENT CONTRACTS (Details) - Schedule of Insurance Contracts [Line Items]
Insurance contracts		13,673	13,984
Life insurance contracts [member] | Reinsurer's share of amount arising from insurance contracts [member]
LIABILITIES ARISING FROM INSURANCE CONTRACTS AND PARTICIPATING INVESTMENT CONTRACTS (Details) - Schedule of Insurance Contracts [Line Items]
Insurance contracts	[1]	(563)	(671)
Life insurance contracts [member] | Reinsurer's share of amount arising from insurance contracts [member] | Insurance contracts [member]
LIABILITIES ARISING FROM INSURANCE CONTRACTS AND PARTICIPATING INVESTMENT CONTRACTS (Details) - Schedule of Insurance Contracts [Line Items]
Insurance contracts	[1]	(563)	(671)
Life insurance contracts [member] | Reinsurer's share of amount arising from insurance contracts [member] | Participating investment contracts [Member]
LIABILITIES ARISING FROM INSURANCE CONTRACTS AND PARTICIPATING INVESTMENT CONTRACTS (Details) - Schedule of Insurance Contracts [Line Items]
Insurance contracts	[1]
Life insurance contracts [member] | Net [Member]
LIABILITIES ARISING FROM INSURANCE CONTRACTS AND PARTICIPATING INVESTMENT CONTRACTS (Details) - Schedule of Insurance Contracts [Line Items]
Insurance contracts		102,267	93,106
Life insurance contracts [member] | Net [Member] | Insurance contracts [member]
LIABILITIES ARISING FROM INSURANCE CONTRACTS AND PARTICIPATING INVESTMENT CONTRACTS (Details) - Schedule of Insurance Contracts [Line Items]
Insurance contracts		88,594	79,122
Life insurance contracts [member] | Net [Member] | Participating investment contracts [Member]
LIABILITIES ARISING FROM INSURANCE CONTRACTS AND PARTICIPATING INVESTMENT CONTRACTS (Details) - Schedule of Insurance Contracts [Line Items]
Insurance contracts		13,673	13,984
Non-life insurance contracts [member] | Gross amount arising from insurance contracts [member]
LIABILITIES ARISING FROM INSURANCE CONTRACTS AND PARTICIPATING INVESTMENT CONTRACTS (Details) - Schedule of Insurance Contracts [Line Items]
Insurance contracts		583	613
Non-life insurance contracts [member] | Gross amount arising from insurance contracts [member] | Unearned premiums [Member]
LIABILITIES ARISING FROM INSURANCE CONTRACTS AND PARTICIPATING INVESTMENT CONTRACTS (Details) - Schedule of Insurance Contracts [Line Items]
Unearned premiums		358	404
Non-life insurance contracts [member] | Gross amount arising from insurance contracts [member] | Claims outstanding [Member]
LIABILITIES ARISING FROM INSURANCE CONTRACTS AND PARTICIPATING INVESTMENT CONTRACTS (Details) - Schedule of Insurance Contracts [Line Items]
Claims outstanding		225	209
Non-life insurance contracts [member] | Reinsurer's share of amount arising from insurance contracts [member]
LIABILITIES ARISING FROM INSURANCE CONTRACTS AND PARTICIPATING INVESTMENT CONTRACTS (Details) - Schedule of Insurance Contracts [Line Items]
Insurance contracts	[1]	(13)	(14)
Non-life insurance contracts [member] | Reinsurer's share of amount arising from insurance contracts [member] | Unearned premiums [Member]
LIABILITIES ARISING FROM INSURANCE CONTRACTS AND PARTICIPATING INVESTMENT CONTRACTS (Details) - Schedule of Insurance Contracts [Line Items]
Unearned premiums	[1]	(13)	(14)
Non-life insurance contracts [member] | Reinsurer's share of amount arising from insurance contracts [member] | Claims outstanding [Member]
LIABILITIES ARISING FROM INSURANCE CONTRACTS AND PARTICIPATING INVESTMENT CONTRACTS (Details) - Schedule of Insurance Contracts [Line Items]
Claims outstanding	[1]
Non-life insurance contracts [member] | Net [Member]
LIABILITIES ARISING FROM INSURANCE CONTRACTS AND PARTICIPATING INVESTMENT CONTRACTS (Details) - Schedule of Insurance Contracts [Line Items]
Insurance contracts		570	599
Non-life insurance contracts [member] | Net [Member] | Unearned premiums [Member]
LIABILITIES ARISING FROM INSURANCE CONTRACTS AND PARTICIPATING INVESTMENT CONTRACTS (Details) - Schedule of Insurance Contracts [Line Items]
Unearned premiums		345	390
Non-life insurance contracts [member] | Net [Member] | Claims outstanding [Member]
LIABILITIES ARISING FROM INSURANCE CONTRACTS AND PARTICIPATING INVESTMENT CONTRACTS (Details) - Schedule of Insurance Contracts [Line Items]
Claims outstanding		£ 225	£ 209

[1] Reinsurance balances are reported within other assets (note 27).