INVESTMENTS - SUMMARY OF INVESTMENTS BY CONTRACTUAL MATURITY (Details) - USD ($) $ in Millions	Jun. 30, 2019	Jan. 01, 2019	Dec. 31, 2018
Amortized cost
Due in one year or less	$ 360.3		$ 405.6
Due after one year through five years	1,169.3		1,346.8
Due after five years through ten years	1,426.0		1,648.2
Due after ten years	10,620.6		9,706.9
Subtotal	13,576.2		13,107.5
Structured securities	5,196.9		5,000.3
Amortized cost	18,773.1		18,107.8
Estimated fair value
Due in one year or less	365.1		409.8
Due after one year through five years	1,216.9		1,377.1
Due after five years through ten years	1,506.7		1,625.7
Due after ten years	11,858.8		9,892.5
Subtotal	14,947.5		13,305.1
Structured securities	5,489.7		5,142.6
Total fixed maturities, available for sale	$ 20,437.2	$ 18,443.7	$ 18,447.7