STOCKHOLDERS' EQUITY (Details) - USD ($)				9 Months Ended
	Aug. 03, 2015	Apr. 07, 2015	Apr. 06, 2015	Sep. 30, 2015	Sep. 30, 2014	Jul. 31, 2015	Dec. 31, 2014
Preferred Stock
Preferred stock, authorized (in shares)				100,000,000			100,000,000
Preferred stock, outstanding (in shares)				250,000			0
Preferred stock, issued (in shares)							0
Preferred stock, par value (in dollars per share)				$ 25.00			$ 25.00
Treasury Stock
Outstanding common stock held by RBS (in shares)	110,461,782
Percent of outstanding common stock held by RBS	20.90%
Cost of stock repurchase				$ 500,000,000	$ 0
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		$ 25.48
Shares repurchased (in shares)				837,853
Cost of stock repurchase		$ 250,000,000		$ 21,000,000
Subordinated Debt | 4.350% fixed rate subordinated debt, due 2025
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			0
Preferred stock, outstanding (in shares)				250,000			0
Preferred stock, issued			$ 250,000,000
Preferred stock, issued (in shares)			250,000	250,000			0
Preferred stock, dividend rate			5.50%
Preferred stock, par value (in dollars per share)			$ 25.00	$ 25.00			$ 0
Preferred stock, liquidation preference (in dollars per share)			$ 1,000	$ 1,000			$ 0
Net proceeds from issuance of stock			$ 247,000,000
Preferred stock, redemption price 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%