STOCKHOLDERS' EQUITY (Details) - USD ($)				3 Months Ended	12 Months Ended
	Aug. 03, 2015	Apr. 07, 2015	Apr. 06, 2015	Mar. 31, 2016	Dec. 31, 2015
Preferred Stock
Preferred stock, authorized (in shares)				100,000,000	100,000,000
Preferred stock, outstanding (in shares)				250,000	250,000
Preferred stock, par value (in dollars per share)				$ 25.00	$ 25.00
RBSG
Treasury Stock
Treasury stock purchased (in shares)	9,615,384	10,473,397
Treasury stock purchased, price per share (in dollars per share)	$ 26.00	$ 23.87
Cost of stock repurchase		$ 250,000,000
4.350% fixed rate subordinated debt, due 2025 | Subordinated Debt
Treasury Stock
Aggregate principal amount of debt	$ 250,000,000
Interest rate	4.35%
Series A Preferred Stock
Preferred Stock
Preferred stock, authorized (in shares)				250,000	250,000
Preferred stock, par value (in dollars per share)			$ 25.00	$ 25.00	$ 25.00
Preferred stock, issued			$ 250,000,000
Preferred stock, issued (in shares)			250,000	250,000	250,000
Preferred stock, dividend rate			5.50%
Preferred stock, liquidation preference (in dollars per share)			$ 1,000	$ 1,000	$ 1,000
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%
Deferred Compensation, Share-based Payments [Member]
Treasury Stock
Treasury stock purchased, price per share (in dollars per share)					$ 25.50
Cost of stock repurchase				$ 0	$ 22,000,000
Stock repurchased during period (in shares)					876,087