INVESTMENTS IN VARIABLE INTEREST ENTITIES - SCHEDULE OF VIEs (Details) - USD ($) $ in Millions	Sep. 30, 2020	Jan. 01, 2020	Dec. 31, 2019	Jan. 01, 2019
Amortized cost
Due in one year or less	$ 289.2		$ 282.2
Due after one year through five years	1,104.1		1,082.2
Due after five years through ten years	1,516.1		1,376.6
Amortized cost	19,783.3		19,179.5
Estimated fair value
Due in one year or less	293.1		286.0
Due after one year through five years	1,164.6		1,130.8
Due after five years through ten years	1,659.4		1,481.7
Total fixed maturities, available for sale	22,702.9	$ 21,293.1	$ 21,295.2	$ 18,443.7
Variable Interest Entity, Primary Beneficiary
Amortized cost
Due in one year or less	4.7
Due after one year through five years	784.1
Due after five years through ten years	434.9
Amortized cost	1,223.7
Estimated fair value
Due in one year or less	3.8
Due after one year through five years	744.7
Due after five years through ten years	424.1
Total fixed maturities, available for sale	$ 1,172.6