DERIVATIVE FINANCIAL INSTRUMENTS (Details) - Schedule of Amount, timing and uncertainty of future cash flows - GBP (£)
£ in Millions

Dec. 31, 2018

Dec. 31, 2017

Fair value hedges [member]

DERIVATIVE FINANCIAL INSTRUMENTS (Details) - Schedule of Amount, timing and uncertainty of future cash flows [Line Items]

Notional (in Pounds)

£ 150,971

£ 109,670

Cash flow hedges [member]

DERIVATIVE FINANCIAL INSTRUMENTS (Details) - Schedule of Amount, timing and uncertainty of future cash flows [Line Items]

Notional (in Pounds)

556,945

£ 549,099

Later than one month and not later than three months [member] | Cash flow hedges [member]

DERIVATIVE FINANCIAL INSTRUMENTS (Details) - Schedule of Amount, timing and uncertainty of future cash flows [Line Items]

Notional (in Pounds)

£ 11,204

Average fixed interest rate

1.03%

Later than one year and not later than five years [member] | Cash flow hedges [member]

DERIVATIVE FINANCIAL INSTRUMENTS (Details) - Schedule of Amount, timing and uncertainty of future cash flows [Line Items]

Notional (in Pounds)

£ 292,712

Average fixed interest rate

1.46%

Later than five years [member] | Cash flow hedges [member]

DERIVATIVE FINANCIAL INSTRUMENTS (Details) - Schedule of Amount, timing and uncertainty of future cash flows [Line Items]

Notional (in Pounds)

£ 181,843

Average fixed interest rate

1.85%

Not later than one month [member] | Cash flow hedges [member]

DERIVATIVE FINANCIAL INSTRUMENTS (Details) - Schedule of Amount, timing and uncertainty of future cash flows [Line Items]

Notional (in Pounds)

£ 4,874

Average fixed interest rate

1.47%

Later than three months and not later than one year [member] | Cash flow hedges [member]

DERIVATIVE FINANCIAL INSTRUMENTS (Details) - Schedule of Amount, timing and uncertainty of future cash flows [Line Items]

Notional (in Pounds)

£ 66,312

Average fixed interest rate

0.99%

Swaps [Member] | Fair value hedges [member]

DERIVATIVE FINANCIAL INSTRUMENTS (Details) - Schedule of Amount, timing and uncertainty of future cash flows [Line Items]

Notional (in Pounds)

£ 490

Swaps [Member] | Cash flow hedges [member]

DERIVATIVE FINANCIAL INSTRUMENTS (Details) - Schedule of Amount, timing and uncertainty of future cash flows [Line Items]

Notional (in Pounds)

10,578

Swaps [Member] | Later than one month and not later than three months [member] | Fair value hedges [member]

DERIVATIVE FINANCIAL INSTRUMENTS (Details) - Schedule of Amount, timing and uncertainty of future cash flows [Line Items]

Notional (in Pounds)

£ 36

Average fixed interest rate

4.82%

Swaps [Member] | Later than one month and not later than three months [member] | Cash flow hedges [member]

DERIVATIVE FINANCIAL INSTRUMENTS (Details) - Schedule of Amount, timing and uncertainty of future cash flows [Line Items]

Notional (in Pounds)

£ 47

Swaps [Member] | Later than one year and not later than five years [member] | Fair value hedges [member]

DERIVATIVE FINANCIAL INSTRUMENTS (Details) - Schedule of Amount, timing and uncertainty of future cash flows [Line Items]

Notional (in Pounds)

£ 283

Average fixed interest rate

5.88%

Swaps [Member] | Later than one year and not later than five years [member] | Cash flow hedges [member]

DERIVATIVE FINANCIAL INSTRUMENTS (Details) - Schedule of Amount, timing and uncertainty of future cash flows [Line Items]

Notional (in Pounds)

£ 2,111

Swaps [Member] | Later than five years [member] | Fair value hedges [member]

DERIVATIVE FINANCIAL INSTRUMENTS (Details) - Schedule of Amount, timing and uncertainty of future cash flows [Line Items]

Notional (in Pounds)

£ 171

Average fixed interest rate

4.44%

Swaps [Member] | Later than five years [member] | Cash flow hedges [member]

DERIVATIVE FINANCIAL INSTRUMENTS (Details) - Schedule of Amount, timing and uncertainty of future cash flows [Line Items]

Notional (in Pounds)

£ 6,119

Swaps [Member] | Not later than one month [member] | Cash flow hedges [member]

DERIVATIVE FINANCIAL INSTRUMENTS (Details) - Schedule of Amount, timing and uncertainty of future cash flows [Line Items]

Notional (in Pounds)

Swaps [Member] | Later than three months and not later than one year [member] | Cash flow hedges [member]

DERIVATIVE FINANCIAL INSTRUMENTS (Details) - Schedule of Amount, timing and uncertainty of future cash flows [Line Items]

Notional (in Pounds)

£ 2,234

Swaps [Member] | Euro to USD | Later than one year and not later than five years [member] | Fair value hedges [member]

DERIVATIVE FINANCIAL INSTRUMENTS (Details) - Schedule of Amount, timing and uncertainty of future cash flows [Line Items]

Average exchange rate

1.13

Swaps [Member] | Euro to USD | Later than one year and not later than five years [member] | Cash flow hedges [member]

DERIVATIVE FINANCIAL INSTRUMENTS (Details) - Schedule of Amount, timing and uncertainty of future cash flows [Line Items]

Average exchange rate

1.10

Swaps [Member] | Euro to USD | Later than five years [member] | Cash flow hedges [member]

DERIVATIVE FINANCIAL INSTRUMENTS (Details) - Schedule of Amount, timing and uncertainty of future cash flows [Line Items]

Average exchange rate

1.07

Swaps [Member] | Euro to USD | Not later than one month [member] | Cash flow hedges [member]

DERIVATIVE FINANCIAL INSTRUMENTS (Details) - Schedule of Amount, timing and uncertainty of future cash flows [Line Items]

Average exchange rate

1.15

Swaps [Member] | Euro to USD | Later than three months and not later than one year [member] | Cash flow hedges [member]

DERIVATIVE FINANCIAL INSTRUMENTS (Details) - Schedule of Amount, timing and uncertainty of future cash flows [Line Items]

Average exchange rate

1.13

Swaps [Member] | USD to Euro | Later than one month and not later than three months [member] | Cash flow hedges [member]

DERIVATIVE FINANCIAL INSTRUMENTS (Details) - Schedule of Amount, timing and uncertainty of future cash flows [Line Items]

Average exchange rate

1.32

Swaps [Member] | USD to Euro | Later than one year and not later than five years [member] | Fair value hedges [member]

DERIVATIVE FINANCIAL INSTRUMENTS (Details) - Schedule of Amount, timing and uncertainty of future cash flows [Line Items]

Average exchange rate

1.30

Swaps [Member] | USD to Euro | Later than one year and not later than five years [member] | Cash flow hedges [member]

DERIVATIVE FINANCIAL INSTRUMENTS (Details) - Schedule of Amount, timing and uncertainty of future cash flows [Line Items]

Average exchange rate

1.27

Swaps [Member] | USD to Euro | Later than five years [member] | Cash flow hedges [member]

DERIVATIVE FINANCIAL INSTRUMENTS (Details) - Schedule of Amount, timing and uncertainty of future cash flows [Line Items]

Average exchange rate

1.28

Swaps [Member] | USD to Euro | Later than three months and not later than one year [member] | Cash flow hedges [member]

DERIVATIVE FINANCIAL INSTRUMENTS (Details) - Schedule of Amount, timing and uncertainty of future cash flows [Line Items]

Average exchange rate

1.34

Swaps [Member] | NOK to GBP | Later than one month and not later than three months [member] | Fair value hedges [member]

DERIVATIVE FINANCIAL INSTRUMENTS (Details) - Schedule of Amount, timing and uncertainty of future cash flows [Line Items]

Average exchange rate

9.22

Swaps [Member] | NOK to GBP | Later than one year and not later than five years [member] | Fair value hedges [member]

DERIVATIVE FINANCIAL INSTRUMENTS (Details) - Schedule of Amount, timing and uncertainty of future cash flows [Line Items]

Average exchange rate

9.19

Swaps [Member] | NOK to GBP | Later than five years [member] | Fair value hedges [member]

DERIVATIVE FINANCIAL INSTRUMENTS (Details) - Schedule of Amount, timing and uncertainty of future cash flows [Line Items]

Average exchange rate

9.03

InterestRateHedges[Member] | Fair value hedges [member]

DERIVATIVE FINANCIAL INSTRUMENTS (Details) - Schedule of Amount, timing and uncertainty of future cash flows [Line Items]

Notional (in Pounds)

£ 150,971

InterestRateHedges[Member] | Later than one month and not later than three months [member] | Fair value hedges [member]

DERIVATIVE FINANCIAL INSTRUMENTS (Details) - Schedule of Amount, timing and uncertainty of future cash flows [Line Items]

Notional (in Pounds)

£ 417

Average fixed interest rate

2.06%

InterestRateHedges[Member] | Later than one year and not later than five years [member] | Fair value hedges [member]

DERIVATIVE FINANCIAL INSTRUMENTS (Details) - Schedule of Amount, timing and uncertainty of future cash flows [Line Items]

Notional (in Pounds)

£ 86,451

Average fixed interest rate

1.75%

InterestRateHedges[Member] | Later than five years [member] | Fair value hedges [member]

DERIVATIVE FINANCIAL INSTRUMENTS (Details) - Schedule of Amount, timing and uncertainty of future cash flows [Line Items]

Notional (in Pounds)

£ 30,834

Average fixed interest rate

2.98%

InterestRateHedges[Member] | Not later than one month [member] | Fair value hedges [member]

DERIVATIVE FINANCIAL INSTRUMENTS (Details) - Schedule of Amount, timing and uncertainty of future cash flows [Line Items]

Notional (in Pounds)

£ 393

Average fixed interest rate

1.38%

InterestRateHedges[Member] | Later than three months and not later than one year [member] | Fair value hedges [member]

DERIVATIVE FINANCIAL INSTRUMENTS (Details) - Schedule of Amount, timing and uncertainty of future cash flows [Line Items]

Notional (in Pounds)

£ 32,876

Average fixed interest rate

1.65%