FAIR VALUE MEASUREMENTS - BALANCE SHEET RECURRING (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Interest sensitive products [Member]
Liabilities:
Fair value, measurement with unobservable inputs reconciliation, beginning balance	$ (1,092.3)	$ (1,057.1)
Purchases, sales, issuances and settlements, net	(69.2)	(4.0)
Total realized and unrealized gains (losses) included in net income	2.3	(34.9)
Total realized and unrealized gains (losses) included in accumulated other comprehensive income (loss)	0.0	0.0
Transfers into level 3	0.0	0.0
Transfers out of level 3	0.0	0.0
Fair value, measurement with unobservable inputs reconciliation, ending balance	(1,159.2)	(1,096.0)
Amount of total gains (losses) for the period included in our net income relating to assets and liabilities still held as of the reporting date	2.3	(34.9)
Corporate securities [Member]
Assets:
Fair value, measurement with unobservable inputs reconciliation, beginning balance	258.5	170.4
Purchases, sales, issuances and settlements, net	4.2	(15.4)
Total realized and unrealized gains (losses) included in net income	0.6	(11.1)
Total realized and unrealized gains (losses) included in accumulated other comprehensive income (loss)	8.0	9.1
Transfers into level 3	17.5	20.3
Transfers out of level 3	0.0	0.0
Fair value, measurement with unobservable inputs reconciliation, ending balance	288.8	173.3
Amount of total gains (losses) for the period included in our net income relating to assets and liabilities still held as of the reporting date	(3.2)	(6.3)
Debt securities issued by foreign governments [Member]
Assets:
Fair value, measurement with unobservable inputs reconciliation, beginning balance	3.9
Purchases, sales, issuances and settlements, net	0.0
Total realized and unrealized gains (losses) included in net income	0.0
Total realized and unrealized gains (losses) included in accumulated other comprehensive income (loss)	0.0
Transfers into level 3	0.0
Transfers out of level 3	0.0
Fair value, measurement with unobservable inputs reconciliation, ending balance	3.9
Amount of total gains (losses) for the period included in our net income relating to assets and liabilities still held as of the reporting date	0.0
Asset-backed Securities [Member]
Assets:
Fair value, measurement with unobservable inputs reconciliation, beginning balance	60.4	35.9
Purchases, sales, issuances and settlements, net	9.9	20.9
Total realized and unrealized gains (losses) included in net income	0.0	0.0
Total realized and unrealized gains (losses) included in accumulated other comprehensive income (loss)	0.4	0.7
Transfers into level 3	0.0	0.0
Transfers out of level 3	(3.5)	(13.7)
Fair value, measurement with unobservable inputs reconciliation, ending balance	67.2	43.8
Amount of total gains (losses) for the period included in our net income relating to assets and liabilities still held as of the reporting date	0.0	0.0
Collateralized debt obligations [Member]
Assets:
Fair value, measurement with unobservable inputs reconciliation, beginning balance	5.4
Purchases, sales, issuances and settlements, net	9.0
Total realized and unrealized gains (losses) included in net income	0.0
Total realized and unrealized gains (losses) included in accumulated other comprehensive income (loss)	0.0
Transfers into level 3	0.0
Transfers out of level 3	(2.9)
Fair value, measurement with unobservable inputs reconciliation, ending balance	11.5
Amount of total gains (losses) for the period included in our net income relating to assets and liabilities still held as of the reporting date	0.0
Commercial mortgage-backed securities [Member]
Assets:
Fair value, measurement with unobservable inputs reconciliation, beginning balance	32.0	1.1
Purchases, sales, issuances and settlements, net	(0.1)	0.0
Total realized and unrealized gains (losses) included in net income	0.0	0.0
Total realized and unrealized gains (losses) included in accumulated other comprehensive income (loss)	0.0	0.0
Transfers into level 3	0.0	0.0
Transfers out of level 3	(17.0)	0.0
Fair value, measurement with unobservable inputs reconciliation, ending balance	14.9	1.1
Amount of total gains (losses) for the period included in our net income relating to assets and liabilities still held as of the reporting date	0.0	0.0
Mortgage pass-through securities [Member]
Assets:
Fair value, measurement with unobservable inputs reconciliation, beginning balance		0.1
Purchases, sales, issuances and settlements, net		(0.1)
Total realized and unrealized gains (losses) included in net income		0.0
Total realized and unrealized gains (losses) included in accumulated other comprehensive income (loss)		0.0
Transfers into level 3		0.0
Transfers out of level 3		0.0
Fair value, measurement with unobservable inputs reconciliation, ending balance		0.0
Amount of total gains (losses) for the period included in our net income relating to assets and liabilities still held as of the reporting date		0.0
Total fixed maturities, available for sale [Member]
Assets:
Fair value, measurement with unobservable inputs reconciliation, beginning balance	360.2	207.5
Purchases, sales, issuances and settlements, net	23.0	5.4
Total realized and unrealized gains (losses) included in net income	0.6	(11.1)
Total realized and unrealized gains (losses) included in accumulated other comprehensive income (loss)	8.4	9.8
Transfers into level 3	17.5	20.3
Transfers out of level 3	(23.4)	(13.7)
Fair value, measurement with unobservable inputs reconciliation, ending balance	386.3	218.2
Amount of total gains (losses) for the period included in our net income relating to assets and liabilities still held as of the reporting date	(3.2)	(6.3)
Equity securities [Member]
Assets:
Fair value, measurement with unobservable inputs reconciliation, beginning balance	25.2	32.0
Purchases, sales, issuances and settlements, net	0.0	1.1
Total realized and unrealized gains (losses) included in net income	0.0	0.0
Total realized and unrealized gains (losses) included in accumulated other comprehensive income (loss)	(0.1)	(0.7)
Transfers into level 3	0.0	0.0
Transfers out of level 3	(0.8)	0.0
Fair value, measurement with unobservable inputs reconciliation, ending balance	24.3	32.4
Amount of total gains (losses) for the period included in our net income relating to assets and liabilities still held as of the reporting date	$ 0.0	0.0
Trading securities [Member]
Assets:
Fair value, measurement with unobservable inputs reconciliation, beginning balance		39.9
Purchases, sales, issuances and settlements, net		(1.1)
Total realized and unrealized gains (losses) included in net income		0.0
Total realized and unrealized gains (losses) included in accumulated other comprehensive income (loss)		0.2
Transfers into level 3		0.0
Transfers out of level 3		0.0
Fair value, measurement with unobservable inputs reconciliation, ending balance		39.0
Amount of total gains (losses) for the period included in our net income relating to assets and liabilities still held as of the reporting date		$ 0.2