FAIR VALUE MEASUREMENTS - FAIR VALUE MEASUREMENTS OF FINANCIAL INSTRUMENTS MEASURED ON A RECURRING BASIS - FINANCIAL INSTRUMENTS (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Cash and cash equivalents:
Held by variable interest entities	$ 69.2	$ 99.6
Fair value, Measurements, Recurring | Estimated fair value
Assets:
Mortgage loans	1,273.6	1,297.5
Policy loans	121.6	120.2
Other invested assets:
Company-owned life insurance	199.1	207.0
Cash and cash equivalents:
Unrestricted	575.7	632.1
Held by variable interest entities	69.2	99.6
Liabilities:
Policyholder account liabilities	14,858.3	13,689.7
Investment borrowings	1,640.5	1,719.6
Borrowings related to variable interest entities	1,066.3	1,144.8
Notes payable – direct corporate obligations	1,077.0	1,283.4
Fair value, Measurements, Recurring | Total amortized cost
Assets:
Mortgage loans	1,411.9	1,218.6
Policy loans	121.6	120.2
Other invested assets:
Company-owned life insurance	199.1	207.0
Cash and cash equivalents:
Unrestricted	575.7	632.1
Held by variable interest entities	69.2	99.6
Liabilities:
Policyholder account liabilities	14,858.3	13,689.7
Investment borrowings	1,639.5	1,715.8
Borrowings related to variable interest entities	1,104.6	1,147.9
Notes payable – direct corporate obligations	1,138.8	1,137.3
Fair value, Measurements, Recurring | Quoted prices in active markets for identical assets or liabilities (Level 1)
Assets:
Mortgage loans	0.0	0.0
Policy loans	0.0	0.0
Other invested assets:
Company-owned life insurance	0.0	0.0
Cash and cash equivalents:
Unrestricted	575.7	632.1
Held by variable interest entities	69.2	99.6
Liabilities:
Policyholder account liabilities	0.0	0.0
Investment borrowings	0.0	0.0
Borrowings related to variable interest entities	0.0	0.0
Notes payable – direct corporate obligations	0.0	0.0
Fair value, Measurements, Recurring | Significant other observable inputs (Level 2)
Assets:
Mortgage loans	0.0	0.0
Policy loans	0.0	0.0
Other invested assets:
Company-owned life insurance	199.1	207.0
Cash and cash equivalents:
Unrestricted	0.0	0.0
Held by variable interest entities	0.0	0.0
Liabilities:
Policyholder account liabilities	0.0	0.0
Investment borrowings	1,640.5	1,719.6
Borrowings related to variable interest entities	1,066.3	1,144.8
Notes payable – direct corporate obligations	1,077.0	1,283.4
Fair value, Measurements, Recurring | Significant unobservable inputs  (Level 3)
Assets:
Mortgage loans	1,273.6	1,297.5
Policy loans	121.6	120.2
Other invested assets:
Company-owned life insurance	0.0	0.0
Cash and cash equivalents:
Unrestricted	0.0	0.0
Held by variable interest entities	0.0	0.0
Liabilities:
Policyholder account liabilities	14,858.3	13,689.7
Investment borrowings	0.0	0.0
Borrowings related to variable interest entities	0.0	0.0
Notes payable – direct corporate obligations	$ 0.0	$ 0.0