INVESTMENT BORROWINGS - TERMS OF THE BORROWINGS FROM THE FHLB (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 1,642.9	$ 1,644.3
Federal Home Loan Bank Advances
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	1,642.9
Federal Home Loan Bank Advances | Borrowings due July 2021 at 0.818%
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 100.0
Interest rate	0.818%
Federal Home Loan Bank Advances | Borrowings due July 2021 at 0.795%
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 100.0
Interest rate	0.795%
Federal Home Loan Bank Advances | Borrowings due August 2021 at 2.550%
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 27.5
Interest rate	2.55%
Federal Home Loan Bank Advances | Borrowings due August 2021 at 0.779%
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 57.7
Interest rate	0.779%
Federal Home Loan Bank Advances | Borrowings due September 2021 at 0.774%
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 50.0
Interest rate	0.774%
Federal Home Loan Bank Advances | Borrowings due May 2022 at 0.596%
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 22.0
Interest rate	0.596%
Federal Home Loan Bank Advances | Borrowings due May 2022 at 0.577%
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 100.0
Interest rate	0.577%
Federal Home Loan Bank Advances | Borrowings due June 2022 at 0.856%
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 10.0
Interest rate	0.856%
Federal Home Loan Bank Advances | Borrowings due July 2022 at 0.636%
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 50.0
Interest rate	0.636%
Federal Home Loan Bank Advances | Borrowings due July 2022 at 0.644%
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 50.0
Interest rate	0.644%
Federal Home Loan Bank Advances | Borrowings due July 2022 at 0.627%
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 50.0
Interest rate	0.627%
Federal Home Loan Bank Advances | Borrowings due August 2022 at 0.632%
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 50.0
Interest rate	0.632%
Federal Home Loan Bank Advances | Borrowings due December 2022 at 0.546%
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 50.0
Interest rate	0.546%
Federal Home Loan Bank Advances | Borrowings due December 2022 at 0.546%, loan 2
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 50.0
Interest rate	0.546%
Federal Home Loan Bank Advances | Borrowings due March 2023 at 2.160%
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 22.6
Interest rate	2.16%
Federal Home Loan Bank Advances | Borrowings due July 2023 at 0.542%
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 50.0
Interest rate	0.542%
Federal Home Loan Bank Advances | Borrowings due July 2023 at 0.541%
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 100.0
Interest rate	0.541%
Federal Home Loan Bank Advances | Borrowings due February 2024 at 0.578%
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 50.0
Interest rate	0.578%
Federal Home Loan Bank Advances | Borrowings due May 2024 at 0.627%
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 50.0
Interest rate	0.627%
Federal Home Loan Bank Advances | Borrowings due May 2024 at 0.636%
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 21.8
Interest rate	0.636%
Federal Home Loan Bank Advances | Borrowings due May 2024 at 0.630%
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 100.0
Interest rate	0.63%
Federal Home Loan Bank Advances | Borrowings due May 2024 at 0.675%
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 50.0
Interest rate	0.675%
Federal Home Loan Bank Advances | Borrowings due June 2024 at 0.543%
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 75.0
Interest rate	0.543%
Federal Home Loan Bank Advances | Borrowings due July 2024 at 0.614%
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 100.0
Interest rate	0.614%
Federal Home Loan Bank Advances | Borrowings due July 2024 at 1.990%
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 15.5
Interest rate	1.99%
Federal Home Loan Bank Advances | Borrowings due July 2024 at 0.764%
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 34.5
Interest rate	0.764%
Federal Home Loan Bank Advances | Borrowings due July 2024 at 0.720%
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 15.0
Interest rate	0.72%
Federal Home Loan Bank Advances | Borrowings due September 2024 at 0.793%
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 25.0
Interest rate	0.793%
Federal Home Loan Bank Advances | Borrowings Due May 2025 at 0.480%
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 21.7
Interest rate	0.48%
Federal Home Loan Bank Advances | Borrowings due June 2025 at 2.940%
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 19.6
Interest rate	2.94%
Federal Home Loan Bank Advances | Borrowings Due September 2025 at .420%
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 125.0
Interest rate	0.42%