RECENTLY ISSUED ACCOUNTING STANDARDS - IMPACT OF ADOPTION (Details) - USD ($) $ in Millions	Sep. 30, 2020	Jun. 30, 2020	Jan. 01, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Jan. 01, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Fixed maturities, available for sale	$ 22,702.9		$ 21,293.1	$ 21,295.2			$ 18,443.7
Mortgage loans	1,444.9		1,559.4	1,566.1
Investments held by variable interest entities	1,172.6		1,178.7	1,188.6
Income tax assets, net	322.0		437.5	432.6			630.9
Reinsurance receivables	4,613.1		4,781.7	4,785.7
Total assets	34,569.2		33,613.1	33,630.9			31,436.7
Retained earnings	657.5		517.9	535.7			193.5
Total shareholders' equity	$ 5,083.9	$ 4,731.2	4,659.2	4,677.0	$ 4,553.3	$ 4,252.2	3,367.8	$ 3,370.9
ASU 2016-13 Credit Losses
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Fixed maturities, available for sale			(2.1)
Mortgage loans			(6.7)
Investments held by variable interest entities			(9.9)
Income tax assets, net			4.9
Reinsurance receivables			(4.0)
Total assets			(17.8)
Retained earnings			(17.8)
Total shareholders' equity			$ (17.8)
ASU 2017-08 Premium Amortization on Purchased Callable Debt Securities
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Fixed maturities, available for sale							(4.0)
Income tax assets, net							0.9
Total assets							(3.1)
Retained earnings							(3.1)
Total shareholders' equity							$ (3.1)
Amounts prior to effect of adoption of authoritative guidance
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Fixed maturities, available for sale				21,295.2				18,447.7
Mortgage loans				1,566.1
Investments held by variable interest entities				1,188.6
Income tax assets, net				432.6				630.0
Reinsurance receivables				4,785.7
Total assets				33,630.9				31,439.8
Retained earnings				535.7				196.6
Total shareholders' equity				$ 4,677.0				$ 3,370.9