STOCKHOLDERS' EQUITY (Details) - USD ($)					3 Months Ended	12 Months Ended
	Aug. 03, 2015	Apr. 07, 2015	Apr. 06, 2015	Oct. 08, 2014	Dec. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Preferred Stock
Preferred stock, authorized (in shares)					100,000,000	100,000,000	100,000,000
Preferred stock, outstanding (in shares)					0	250,000	0
Preferred stock, par value (in dollars per share)					$ 25.00	$ 25.00	$ 25.00
Preferred stock, issued (in shares)					0		0
Treasury Stock
Cost of stock repurchase						$ 500,000,000	$ 334,000,000	$ 0
Long-term borrowed funds					$ 4,642,000,000	$ 9,886,000,000	4,642,000,000
Deferred compensation, share-based payments
Treasury Stock
Treasury stock purchased, price per share (in dollars per share)					$ 25.03	$ 25.50
Cost of stock repurchase					$ 2,000,000	$ 22,000,000
Shares repurchased (in shares)					80,358	876,087
RBSG
Treasury Stock
Treasury stock purchased (in shares)	9,615,384	10,473,397				20,088,781
Treasury stock purchased, price per share (in dollars per share)	$ 26.00	$ 23.87		$ 23.36
Cost of stock repurchase		$ 250,000,000		$ 334,000,000
Shares repurchased (in shares)				14,297,761
Long-term borrowed funds					$ 2,000,000,000	$ 1,250,000,000	$ 2,000,000,000
4.350% fixed rate subordinated debt, due 2025 | Subordinated Debt
Treasury Stock
Aggregate principal amount of debt	$ 250,000,000
Interest rate	4.35%
4.082% fixed rate subordinated debt, due 2025 | Subordinated Debt | RBSG
Treasury Stock
Interest rate				4.082%	4.082%	4.082%	4.082%
Long-term borrowed funds				$ 334,000,000	$ 334,000,000	$ 334,000,000	$ 334,000,000
Debt term				10 years
Series A Preferred Stock
Preferred Stock
Preferred stock, authorized (in shares)					0	250,000	0
Preferred stock, outstanding (in shares)					0	250,000	0
Preferred stock, par value (in dollars per share)			$ 25.00		$ 0	$ 25.00	$ 0
Preferred stock, issued			$ 250,000,000
Preferred stock, issued (in shares)			250,000		0	250,000	0
Preferred stock, dividend rate			5.50%
Preferred stock, liquidation preference (in dollars per share)			$ 1,000		$ 0	$ 1,000	$ 0
Net proceeds from issuance of stock			$ 247,000,000
Preferred stock, redemption price per share (in dollars per share)			$ 1,000
Series A Preferred Stock | LIBOR
Preferred Stock
Preferred stock, dividend payment rate, basis spread on variable rate, beginning after April 6, 2020			3.96%