FAIR VALUE OF OTHER FINANCIAL INSTRUMENTS - FAIR VALUE MEASUREMENT AT REPORTING DATE (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Assets, Fair Value Disclosure [Abstract]
Estimated Fair Value	$ 1,353.2	$ 1,069.7
Derivative assets	18.0	13.3
Separate Accounts assets	2,704.2	2,280.8
Liabilities, Fair Value Disclosure [Abstract]
Derivative Liabilities	2.9	0.9
Industrial and Miscellaneous
Assets, Fair Value Disclosure [Abstract]
Estimated Fair Value	1,394.8	1,000.7
Estimate of Fair Value Measurement
Assets, Fair Value Disclosure [Abstract]
Estimated Fair Value	0.0	0.0
Common stocks	0.2	2.0
Derivative assets	15.1	12.4
Separate Accounts assets	2,263.2	1,850.0
Total Assets	2,278.5	1,864.4
Liabilities, Fair Value Disclosure [Abstract]
Derivative Liabilities	0.0	0.0
Total Liabilities at Fair Value	0.0	0.0
Estimate of Fair Value Measurement \| Level 1
Assets, Fair Value Disclosure [Abstract]
Estimated Fair Value	0.0	0.0
Common stocks	0.0	0.0
Derivative assets	0.0	0.0
Separate Accounts assets	2,248.9	1,835.6
Total Assets	2,248.9	1,835.6
Liabilities, Fair Value Disclosure [Abstract]
Derivative Liabilities	0.0	0.0
Total Liabilities at Fair Value	0.0	0.0
Estimate of Fair Value Measurement \| Level 2
Assets, Fair Value Disclosure [Abstract]
Estimated Fair Value	0.0	0.0
Common stocks	0.0	0.0
Derivative assets	15.1	12.4
Separate Accounts assets	14.3	14.4
Total Assets	29.4	26.8
Liabilities, Fair Value Disclosure [Abstract]
Derivative Liabilities	0.0	0.0
Total Liabilities at Fair Value	0.0	0.0
Estimate of Fair Value Measurement \| Level 3
Assets, Fair Value Disclosure [Abstract]
Estimated Fair Value	0.0	0.0
Common stocks	0.2	2.0
Derivative assets	0.0	0.0
Separate Accounts assets	0.0	0.0
Total Assets	0.2	2.0
Liabilities, Fair Value Disclosure [Abstract]
Derivative Liabilities	0.0	0.0
Total Liabilities at Fair Value	0.0	0.0
Estimate of Fair Value Measurement \| Net Asset Value (NAV)
Assets, Fair Value Disclosure [Abstract]
Estimated Fair Value	0.0	0.0
Common stocks	0.0	0.0
Derivative assets	0.0	0.0
Separate Accounts assets	0.0	0.0
Total Assets	0.0	0.0
Liabilities, Fair Value Disclosure [Abstract]
Derivative Liabilities	0.0	0.0
Total Liabilities at Fair Value	0.0	0.0
Estimate of Fair Value Measurement \| Commercial mortgage-backed
Assets, Fair Value Disclosure [Abstract]
Estimated Fair Value	0.0	0.0
Estimate of Fair Value Measurement \| Commercial mortgage-backed \| Level 1
Assets, Fair Value Disclosure [Abstract]
Estimated Fair Value	0.0	0.0
Estimate of Fair Value Measurement \| Commercial mortgage-backed \| Level 2
Assets, Fair Value Disclosure [Abstract]
Estimated Fair Value	0.0	0.0
Estimate of Fair Value Measurement \| Commercial mortgage-backed \| Level 3
Assets, Fair Value Disclosure [Abstract]
Estimated Fair Value	0.0	0.0
Estimate of Fair Value Measurement \| Commercial mortgage-backed \| Net Asset Value (NAV)
Assets, Fair Value Disclosure [Abstract]
Estimated Fair Value	0.0	0.0
Estimate of Fair Value Measurement \| Industrial and Miscellaneous
Assets, Fair Value Disclosure [Abstract]
Common stocks	0.2	2.0
Estimate of Fair Value Measurement \| Industrial and Miscellaneous \| Level 1
Assets, Fair Value Disclosure [Abstract]
Common stocks	0.0	0.0
Estimate of Fair Value Measurement \| Industrial and Miscellaneous \| Level 2
Assets, Fair Value Disclosure [Abstract]
Common stocks	0.0	0.0
Estimate of Fair Value Measurement \| Industrial and Miscellaneous \| Level 3
Assets, Fair Value Disclosure [Abstract]
Common stocks	0.2	2.0
Estimate of Fair Value Measurement \| Industrial and Miscellaneous \| Net Asset Value (NAV)
Assets, Fair Value Disclosure [Abstract]
Common stocks	0.0	0.0
Estimate of Fair Value Measurement \| Options
Assets, Fair Value Disclosure [Abstract]
Derivative assets	15.1	12.4
Estimate of Fair Value Measurement \| Options \| Level 1
Assets, Fair Value Disclosure [Abstract]
Derivative assets	0.0	0.0
Estimate of Fair Value Measurement \| Options \| Level 2
Assets, Fair Value Disclosure [Abstract]
Derivative assets	15.1	12.4
Estimate of Fair Value Measurement \| Options \| Level 3
Assets, Fair Value Disclosure [Abstract]
Derivative assets	0.0	0.0
Estimate of Fair Value Measurement \| Options \| Net Asset Value (NAV)
Assets, Fair Value Disclosure [Abstract]
Derivative assets	$ 0.0	$ 0.0

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

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Amount of asset at fair value held for benefit of separate account policyholder.

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