FAIR VALUE - Unobservable Inputs Information (Details) - Mortgage servicing rights - Discounted cash flow - Unobservable inputs Level 3	Sep. 30, 2021	Dec. 31, 2020
Constant prepayment rate | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unobservable input (as a percent)	0.111	0.124
Constant prepayment rate | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unobservable input (as a percent)	0.296	0.280
Constant prepayment rate | Weighted Average
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unobservable input (as a percent)	0.200	0.211
Discount rate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unobservable input (as a percent)	0.120	0.120
Discount rate | Weighted Average
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unobservable input (as a percent)	0.120	0.120