INVESTMENTS - SUMMARY OF INVESTMENTS BY CONTRACTUAL MATURITY (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
Due after ten years	11,390.5		10,908.6
Subtotal	14,299.9		13,649.6
Structured securities	5,483.4		5,529.9
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
Due after ten years	13,828.3		12,583.7
Subtotal	16,945.4		15,482.2
Structured securities	5,757.5		5,813.0
Total fixed maturities, available for sale	$ 22,702.9	$ 21,293.1	$ 21,295.2	$ 18,443.7