GOODWILL AND INTANGIBLE ASSETS - Narrative (Details) $ in Millions	12 Months Ended			372 Months Ended
	Dec. 31, 2018 USD ($) reporting_unit	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2018 USD ($) business	Aug. 01, 2018 USD ($)
Goodwill [Line Items]
Number of reporting units | reporting_unit	2
Number of acquisitions of banks or assets of banks | business				25
Goodwill acquired during period	$ 36
Goodwill impairment	0	$ 0	$ 0
Amortization of intangible assets	3	0	$ 0
Consumer Banking
Goodwill [Line Items]
Goodwill accumulated impairment loss	5,900	5,900		$ 5,900
Commercial Banking
Goodwill [Line Items]
Goodwill accumulated impairment loss	50	$ 50		50
Franklin American Mortgage Company
Goodwill [Line Items]
Intangible assets acquired	30			30	$ 32
Amortization expense	$ 2			$ 2