INVESTMENT BORROWINGS (Details) $ in Millions	9 Months Ended
	Sep. 30, 2018 USD ($) subsidiary	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Number of insurance subsidiaries that are members of the FHLB | subsidiary	3
Investment borrowings	$ 1,646.1		$ 1,646.7
Federal Home Loan Bank advances
Schedule of Trading Securities and Other Trading Assets [Line Items]
Federal home loan bank stock	71.1
Investment borrowings	1,646.1
Federal home loan bank, advances, collateral pledged	2,000.0
Aggregate fee to prepay all fixed rate FHLB borrowings	1.0
Interest expense on FHLB borrowings	30.3	$ 19.5
Federal Home Loan Bank advances | Borrowings due February 2019
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment borrowings	$ 50.0
Interest rate	2.405%
Federal Home Loan Bank advances | Borrowings due July 2019
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment borrowings	$ 21.8
Interest rate	2.672%
Federal Home Loan Bank advances | Borrowings due October 2019
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment borrowings	$ 15.0
Interest rate	2.85%
Federal Home Loan Bank advances | Borrowing due May 2020
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment borrowings	$ 50.0
Interest rate	2.678%
Federal Home Loan Bank advances | Borrowings due June 2020
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment borrowings	$ 21.8
Interest rate	1.96%
Federal Home Loan Bank advances | Borrowings due September 2020
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment borrowings	$ 25.0
Interest rate	2.991%
Federal Home Loan Bank advances | Borrowings due September 2020 rate two
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment borrowings	$ 100.0
Interest rate	2.876%
Federal Home Loan Bank advances | Borrowings due September 2020 rate three
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment borrowings	$ 50.0
Interest rate	2.902%
Federal Home Loan Bank advances | Borrowings due September 2020 rate four
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment borrowings	$ 75.0
Interest rate	2.506%
Federal Home Loan Bank advances | Borrowings due October 2020
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment borrowings	$ 100.0
Interest rate	2.453%
Federal Home Loan Bank advances | Borrowings due December 2020
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment borrowings	$ 50.0
Interest rate	2.814%
Federal Home Loan Bank advances | Borrowings due July 2021
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment borrowings	$ 100.0
Interest rate	2.889%
Federal Home Loan Bank advances | Borrowings due July 2021 rate two
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment borrowings	$ 100.0
Interest rate	2.859%
Federal Home Loan Bank advances | Borrowings due August 2021
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment borrowings	$ 28.2
Interest rate	2.55%
Federal Home Loan Bank advances | Borrowings due August 2021 rate two
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment borrowings	$ 57.7
Interest rate	2.871%
Federal Home Loan Bank advances | Borrowings due August 2021 Rate Three
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment borrowings	$ 125.0
Interest rate	2.696%
Federal Home Loan Bank advances | Borrowings due September 2021
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment borrowings	$ 50.0
Interest rate	2.851%
Federal Home Loan Bank advances | Borrowings due May 2022, Rate One
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment borrowings	$ 22.0
Interest rate	2.665%
Federal Home Loan Bank advances | Borrowings due May 2022, Rate Two
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment borrowings	$ 100.0
Interest rate	2.505%
Federal Home Loan Bank advances | Borrowings due June 2022
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment borrowings	$ 10.0
Interest rate	2.941%
Federal Home Loan Bank advances | Borrowings due July 2022 rate one
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment borrowings	$ 50.0
Interest rate	2.703%
Federal Home Loan Bank advances | Borrowings due July 2022 rate two
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment borrowings	$ 50.0
Interest rate	2.722%
Federal Home Loan Bank advances | Borrowings due July 2022 rate three
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment borrowings	$ 50.0
Interest rate	2.719%
Federal Home Loan Bank advances | Borrowings due August 2022
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment borrowings	$ 50.0
Interest rate	2.733%
Federal Home Loan Bank advances | Borrowings due December 2022, variable rate 2.621%
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment borrowings	$ 50.0
Interest rate	2.621%
Federal Home Loan Bank advances | Borrowings due December 2022, variable rate 2.621%, loan 2
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment borrowings	$ 50.0
Interest rate	2.621%
Federal Home Loan Bank advances | Borrowings due March 2023
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment borrowings	$ 24.1
Interest rate	2.16%
Federal Home Loan Bank advances | Borrowings Due July 2023 Rate One
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment borrowings	$ 50.0
Interest rate	2.548%
Federal Home Loan Bank advances | Borrowings Due July 2023 Rate Two
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment borrowings	$ 100.0
Interest rate	2.548%
Federal Home Loan Bank advances | Borrowings due June 2025
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment borrowings	$ 20.5
Interest rate	2.94%