RELATED PARTY TRANSACTIONS AND SIGNIFICANT TRANSACTIONS WITH RBS - Narrative (Details) - USD ($) $ in Millions			12 Months Ended
		Dec. 03, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Nov. 30, 2015
Related Party Transaction [Line Items]
Repayments of long-term borrowed funds			$ 766	$ 6	$ 291
Gains (losses) on extinguishment of debt		$ 3	3	0	0
Long-term borrowed funds			9,886	4,642
Payments of ordinary common stock dividends			214	806	1,200
Dividends Common Stock Exchange Transaction				666	1,000
RBSG
Related Party Transaction [Line Items]
Repayments of long-term borrowed funds		$ 750	750	0	280
Long-term borrowed funds			1,250	2,000
Payments of ordinary common stock dividends			71	124	185
Executive Officers, Family Members, and Their Businesses
Related Party Transaction [Line Items]
Related party loans			136	126
Special Dividend | RBSG
Related Party Transaction [Line Items]
Dividends Common Stock Exchange Transaction				666	$ 1,000
Interest rate swaps | Derivatives not designated as hedging instruments
Related Party Transaction [Line Items]
Notional amount	[1]		33,719	31,848
Interest rate swaps | Derivatives not designated as hedging instruments | RBSG
Related Party Transaction [Line Items]
Notional amount			6,500	9,800
Foreign exchange contracts | RBSG
Related Party Transaction [Line Items]
Notional amount			3,600	4,700
Foreign exchange contracts | Derivatives not designated as hedging instruments
Related Party Transaction [Line Items]
Notional amount	[1]		8,366	$ 8,359
Citizens Financial Group, Inc.
Related Party Transaction [Line Items]
Long-term borrowed funds			$ 2,595
Subordinated Debt | Citizens Financial Group, Inc. | Purchase commitment
Related Party Transaction [Line Items]
Long-term borrowed funds						$ 500

[1] The notional or contractual amount of interest rate derivatives and foreign exchange contracts is the amount upon which interest and other payments under the contract are based. Notional amounts are typically not exchanged. Therefore, notional amounts should not be taken as the measure of credit or market risk, as they tend to greatly overstate the true economic risk of these contracts.