REGULATORY MATTERS - Narrative (Details) - USD ($)	3 Months Ended				12 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common stock, dividends per share (in dollars per share)			$ 0.27	$ 0.27
Preferred stock, issued			$ 840,000,000		$ 840,000,000	$ 247,000,000
Treasury stock purchased					1,025,000,000	820,000,000	$ 430,000,000
Common stock dividends					471,000,000	322,000,000	241,000,000
Dividends to preferred stockholders					29,000,000	14,000,000	$ 14,000,000
Forecast
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common stock, dividends per share (in dollars per share)	$ 0.32	$ 0.32
Repurchases of stock	$ 1,020,000,000.00
Series B
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Preferred stock, issued			296,000,000		296,000,000
Dividends to preferred stockholders					11,000,000
Series C
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Preferred stock, issued			297,000,000		297,000,000
Dividends to preferred stockholders					4,000,000
Series A
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Preferred stock, issued			247,000,000		247,000,000	247,000,000
Dividends to preferred stockholders					14,000,000	$ 14,000,000
5.158% fixed-to-floating rate subordinated debt, callable, due 2023
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Debt redeemed			$ 333,000,000		$ 333,000,000
Interest rate			5.158%		5.158%