RELATED PARTY TRANSACTIONS - Narrative (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Related Party Transaction [Line Items]
Dividend to parent				$ 90	$ 0
Dividends Common Stock Exchange Transaction					666
Interest rate swaps | Derivatives not designated as hedging instruments
Related Party Transaction [Line Items]
Notional amount	[1]	$ 31,628		31,628		$ 31,848
Foreign exchange contracts | Derivatives not designated as hedging instruments
Related Party Transaction [Line Items]
Notional amount	[1]	7,783		7,783		8,359
RBS
Related Party Transaction [Line Items]
Dividend to parent		11	$ 50	71	85
RBS | Interest rate swaps | Derivatives not designated as hedging instruments
Related Party Transaction [Line Items]
Notional amount		7,200		7,200		9,800
RBS | Foreign exchange contracts
Related Party Transaction [Line Items]
Notional amount		4,100		4,100		4,700
Executive Officers, Family Members, and Their Businesses
Related Party Transaction [Line Items]
Related party loans		$ 135		$ 135		$ 126
Special Dividend [Member] | RBS
Related Party Transaction [Line Items]
Dividends Common Stock Exchange Transaction			$ 333		$ 666

[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. For interest rate derivatives, the notional amount is 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.