SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Stable Value Product Account Balances (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Summarized financial information for the company's segments
Stable value product account balances marketed through structured programs	$ 4,083.8	$ 3,337.9
Future maturities of stable value products
2019	1,253.9
2020 - 2021	3,042.5
2022 - 2023	818.7
Thereafter	$ 118.7
Minimum
Summarized financial information for the company's segments
Maturities of GICs and funding agreements	1 year
Maximum
Summarized financial information for the company's segments
Maturities of GICs and funding agreements	12 years