SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - IMPACT OF ADOPTION (Details) - USD ($) $ in Millions	Dec. 31, 2019	Jan. 01, 2019	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017	Jan. 01, 2017	Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Fixed maturities, available for sale securities	$ 21,295.2	$ 18,443.7	$ 18,447.7
Income tax assets, net	432.6		630.0
Income tax assets, net		630.9
Total assets	33,630.9	31,436.7	31,439.8			$ 31,975.5
Accumulated other comprehensive income	1,372.5		177.7	$ 1,195.8
Retained earnings (accumulated deficit)	535.7	193.5	196.6	576.7		650.1
Stockholders' equity	$ 4,677.0	3,367.8	3,370.9	4,847.5	$ 4,847.5	$ 4,487.2	$ 4,486.9
Amounts prior to effect of adoption of authoritative guidance
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Fixed maturities, available for sale securities			18,447.7
Income tax assets, net			630.0
Total assets			31,439.8				31,975.2
Accumulated other comprehensive income				1,212.1
Retained earnings (accumulated deficit)			196.6	560.4			650.7
Stockholders' equity			$ 3,370.9	4,847.5			$ 4,486.9
Accounting Standards Update 2016-01 | Restatement adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accumulated other comprehensive income				(16.3)
Retained earnings (accumulated deficit)				16.3
Stockholders' equity				$ 0.0
Accounting Standards Update 2017-08 | Restatement adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Fixed maturities, available for sale securities		(4.0)
Income tax assets, net		0.9
Total assets		(3.1)
Retained earnings (accumulated deficit)		(3.1)
Stockholders' equity		$ (3.1)