SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Premises and Equipment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Other Assets
Property, Plant and Equipment [Line Items]
Premises held-for-sale	$ 5.2	$ 1.0
Buildings and improvements, including leasehold improvements | Minimum
Property, Plant and Equipment [Line Items]
Useful life	10 years
Buildings and improvements, including leasehold improvements | Maximum
Property, Plant and Equipment [Line Items]
Useful life	40 years
Furniture and equipment | Minimum
Property, Plant and Equipment [Line Items]
Useful life	3 years
Furniture and equipment | Maximum
Property, Plant and Equipment [Line Items]
Useful life	15 years