SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (RECENTLY ADOPTED ACCOUNTING PRONOUNCEMENT CASH FLOW) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net investment income	$ 1,321.2	$ 1,256.3	$ 1,213.9
Cash flows from operating activities:
Other operating costs	(788.5)	(747.4)	(747.9)
Cash flows from operating activities	317.8	633.3	775.7
Cash flows from investing activities:
Sales of investments	3,210.2	2,460.7	2,828.9
Change in cash and cash equivalents held by variable interest entities		0.0	0.0
Other	(25.0)	(23.4)	(22.5)
Net cash provided (used) by investing activities	(525.7)	(270.2)	(930.4)
Net increase in cash and cash equivalents	(100.7)	89.1	(128.5)
Cash and cash equivalents - unrestricted and held by variable interest entities, beginning of period	757.3	668.2	796.7
Cash and cash equivalents - unrestricted and held by variable interest entities, end of period		757.3	668.2
Accounting Standards Update 2016-18
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net investment income		0.0	0.0
Cash flows from operating activities:
Other operating costs		0.0
Cash flows from operating activities		0.0	0.0
Cash flows from investing activities:
Sales of investments		0.0	0.0
Change in cash and cash equivalents held by variable interest entities		(10.4)	(175.1)
Other		0.0
Net cash provided (used) by investing activities		(10.4)	(175.1)
Net increase in cash and cash equivalents		(10.4)	(175.1)
Cash and cash equivalents - unrestricted and held by variable interest entities, beginning of period	178.9	189.3	364.4
Cash and cash equivalents - unrestricted and held by variable interest entities, end of period		178.9	189.3
Accounting Standards Update 2017-15, COLI Death Benefits
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net investment income		0.0
Cash flows from operating activities:
Other operating costs		(6.5)
Cash flows from operating activities		(6.5)
Cash flows from investing activities:
Sales of investments		0.0
Change in cash and cash equivalents held by variable interest entities		0.0
Other		6.5
Net cash provided (used) by investing activities		6.5
Net increase in cash and cash equivalents		0.0
Cash and cash equivalents - unrestricted and held by variable interest entities, beginning of period	0.0	0.0
Cash and cash equivalents - unrestricted and held by variable interest entities, end of period		0.0	0.0
Accounting Standards Update 2017-15, Equity Method Investments
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net investment income		26.7	12.9
Cash flows from operating activities:
Other operating costs		0.0
Cash flows from operating activities		26.7	12.9
Cash flows from investing activities:
Sales of investments		(26.7)	(12.9)
Change in cash and cash equivalents held by variable interest entities		0.0	0.0
Other		0.0
Net cash provided (used) by investing activities		(26.7)	(12.9)
Net increase in cash and cash equivalents		0.0	0.0
Cash and cash equivalents - unrestricted and held by variable interest entities, beginning of period	0.0	0.0	0.0
Cash and cash equivalents - unrestricted and held by variable interest entities, end of period		0.0	0.0
Previously Reported
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net investment income		1,229.6	1,201.0
Cash flows from operating activities:
Other operating costs		(740.9)
Cash flows from operating activities		613.1	762.8
Cash flows from investing activities:
Sales of investments		2,487.4	2,841.8
Change in cash and cash equivalents held by variable interest entities		10.4	175.1
Other		(29.9)
Net cash provided (used) by investing activities		(239.6)	(742.4)
Net increase in cash and cash equivalents		99.5	46.6
Cash and cash equivalents - unrestricted and held by variable interest entities, beginning of period	$ 578.4	478.9	432.3
Cash and cash equivalents - unrestricted and held by variable interest entities, end of period		$ 578.4	$ 478.9