INVESTMENTS IN VARIABLE INTEREST ENTITIES - SCHEDULE OF VIEs (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020	Jan. 01, 2020
Amortized cost
Due after one year through five years	$ 1,081.0	$ 987.4
Due after five years through ten years	1,440.6	1,540.4
Amortized cost	20,698.0	19,921.1
Estimated fair value
Due after one year through five years	1,150.7	1,052.9
Due after five years through ten years	1,584.1	1,715.6
Fixed maturities, available for sale, at fair value (net of allowance for credit losses: June 30, 2021 - $2.3 and December 31, 2020 - $2.2; amortized cost: June 30, 2021 - $20,698.0 and December 31, 2020 - $19,921.1)	23,806.6	$ 23,383.6	$ 21,293.1
Variable Interest Entity, Primary Beneficiary
Amortized cost
Due after one year through five years	634.0
Due after five years through ten years	605.0
Amortized cost	1,239.0
Estimated fair value
Due after one year through five years	628.7
Due after five years through ten years	604.8
Fixed maturities, available for sale, at fair value (net of allowance for credit losses: June 30, 2021 - $2.3 and December 31, 2020 - $2.2; amortized cost: June 30, 2021 - $20,698.0 and December 31, 2020 - $19,921.1)	$ 1,233.5

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in sixth through tenth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in second through fifth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

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Amortized cost excluding accrued interest, before allowance for credit loss, 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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Amount excluding accrued interest, 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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