REGULATORY MATTERS - Narrative (Details) $ / shares in Units, $ in Millions			3 Months Ended				6 Months Ended
	Jun. 29, 2018 USD ($)	May 24, 2018 USD ($) $ / shares shares	Mar. 31, 2019 $ / shares	Sep. 30, 2018 $ / shares	Jun. 30, 2018 USD ($) subsidiary $ / shares	Jun. 30, 2017 USD ($) $ / shares	Jun. 30, 2018 USD ($) subsidiary $ / shares	Jun. 30, 2017 USD ($) $ / shares	Jun. 30, 2019 USD ($)	Dec. 31, 2017 $ / shares
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common share dividends (in dollars per share) | $ / shares					$ 0.22	$ 0.14	$ 0.44	$ 0.28
Debt redeemed							$ 7,584	$ 9,751
Preferred stock, par value (in dollars per share) | $ / shares					$ 25.00		$ 25.00			$ 25.00
Common share dividends					$ 107	$ 71	$ 215	143
Preferred share dividends					7	7	7	7
Treasury stock purchased					$ 150	$ 130	$ 325	$ 260
Banking Subsidiaries
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Number of financial subsidiaries | subsidiary					2		2
Scenario, Forecast
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common share dividends (in dollars per share) | $ / shares			$ 0.32	$ 0.27
Common share repurchases, authorized amount									$ 1,020
5.158% fixed-to-floating rate callable subordinated debt | Subordinated Debt | Parent Company
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Debt redeemed	$ 333
Interest rate	5.158%
Noncumulative Preferred Stock
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Preferred stock issued (in shares) | shares		300,000
Preferred stock, dividend rate		6.00%
Preferred stock, par value (in dollars per share) | $ / shares		$ 25
Preferred stock, liquidation preference per share (in dollars per share) | $ / shares		$ 1,000
Carrying amount of preferred stock		$ 296