INVESTMENT BORROWINGS - TERMS OF THE BORROWINGS FROM THE FHLB (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 1,645.2	$ 1,645.8
Federal Home Loan Bank advances
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	1,645.2
Federal Home Loan Bank advances | Borrowings due July 2019
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 50.0
Interest rate	3.131%
Federal Home Loan Bank advances | Borrowings due October 2019
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 15.0
Interest rate	3.095%
Federal Home Loan Bank advances | Borrowings due June 2020
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 21.8
Interest rate	1.96%
Federal Home Loan Bank advances | Borrowings due September 2020
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 25.0
Interest rate	2.968%
Federal Home Loan Bank advances | Borrowings due October 2020
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 100.0
Interest rate	2.708%
Federal Home Loan Bank advances | Borrowings due July 2021 at 3.147%
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 100.0
Interest rate	3.147%
Federal Home Loan Bank advances | Borrowings due July 2021 at 3.117%
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 100.0
Interest rate	3.117%
Federal Home Loan Bank advances | Borrowings due August 2021 at 2.550%
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 28.2
Interest rate	2.55%
Federal Home Loan Bank advances | Borrowings due August 2021 at 3.095%
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 57.7
Interest rate	3.095%
Federal Home Loan Bank advances | Borrowings due August 2021 at 2.884%
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 125.0
Interest rate	2.884%
Federal Home Loan Bank advances | Borrowings due September 2021
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 50.0
Interest rate	3.061%
Federal Home Loan Bank advances | Borrowings due May 2022 at 2.874%
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 22.0
Interest rate	2.874%
Federal Home Loan Bank advances | Borrowings due May 2022 at 2.859%
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 100.0
Interest rate	2.859%
Federal Home Loan Bank advances | Borrowings due June 2022
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 10.0
Interest rate	3.067%
Federal Home Loan Bank advances | Borrowings due July 2022 at 2.951%
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 50.0
Interest rate	2.951%
Federal Home Loan Bank advances | Borrowings due July 2022 at 2.961%
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 50.0
Interest rate	2.961%
Federal Home Loan Bank advances | Borrowings due July 2022 at 2.962%
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 50.0
Interest rate	2.962%
Federal Home Loan Bank advances | Borrowings due August 2022
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 50.0
Interest rate	2.955%
Federal Home Loan Bank advances | Borrowings due December 2022 at 2.820%
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 50.0
Interest rate	2.82%
Federal Home Loan Bank advances | Borrowings due December 2022 at 2.820%, loan 2
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 50.0
Interest rate	2.82%
Federal Home Loan Bank advances | Borrowings due March 2023
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 23.6
Interest rate	2.16%
Federal Home Loan Bank advances | Borrowings due July 2023 at 2.784%
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 50.0
Interest rate	2.784%
Federal Home Loan Bank advances | Borrowings due July 2023 at 2.784%, loan 2
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 100.0
Interest rate	2.784%
Federal Home Loan Bank advances | Borrowings due February 2024
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 50.0
Interest rate	2.83%
Federal Home Loan Bank advances | Borrowings due May 2024 at 2.869%
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 50.0
Interest rate	2.869%
Federal Home Loan Bank advances | Borrowings due May 2024 at 2.863%
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 21.8
Interest rate	2.863%
Federal Home Loan Bank advances | Borrowings due May 2024 at 2.887%
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 100.0
Interest rate	2.887%
Federal Home Loan Bank advances | Borrowings due May 2024 at 2.932%
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 50.0
Interest rate	2.932%
Federal Home Loan Bank advances | Borrowings due June 2024
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 75.0
Interest rate	2.64%
Federal Home Loan Bank advances | Borrowings due June 2025
Debt and Equity Securities, FV-NI [Line Items]
Investment borrowings	$ 20.1
Interest rate	2.94%