COMMITMENTS AND CONTINGENCIES - OTHER LITIGATION (Details) £ in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2015 GBP (£)	Sep. 30, 2017 USD ($)	Oct. 01, 2014 GBP (£)
Loss Contingencies [Line Items]
Investment in RBS Sempra Commodities LLP | $		$ 67
HMRC VAT Claim [Member]
Loss Contingencies [Line Items]
VAT tax claim paid upon appeal			£ 86
Filed claims amount	£ 160
Plaintiffs [Member] | HMRC VAT Claim [Member]
Loss Contingencies [Line Items]
Filed claims amount	£ 80