FAIR VALUE MEASUREMENTS - BALANCE SHEET RECURRING (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Interest sensitive products
Liabilities:
Fair value, measurement with unobservable inputs reconciliation, beginning balance	$ (1,315.4)	$ (1,159.2)	$ (1,334.8)	$ (1,092.3)
Purchases, sales, issuances and settlements, net	(33.9)	(48.7)	(51.5)	(117.9)
Total realized and unrealized gains (losses) included in net income	16.0	2.5	53.0	4.8
Total realized and unrealized gains (losses) included in accumulated other comprehensive income (loss)	0.0	0.0	0.0	0.0
Transfers into level 3	0.0	0.0	0.0	0.0
Transfers out of level 3	0.0	0.0	0.0	0.0
Fair value, measurement with unobservable inputs reconciliation, ending balance	(1,333.3)	(1,205.4)	(1,333.3)	(1,205.4)
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	16.0	2.5	53.0	4.8
Corporate securities
Assets:
Fair value, measurement with unobservable inputs reconciliation, beginning balance	200.1	288.8	230.4	258.5
Purchases, sales, issuances and settlements, net	(6.9)	(18.3)	3.7	(14.1)
Total realized and unrealized gains (losses) included in net income	0.1	7.2	1.3	7.8
Total realized and unrealized gains (losses) included in accumulated other comprehensive income (loss)	(0.8)	(8.9)	(3.2)	(0.5)
Transfers into level 3	4.4	11.6	0.0	11.6
Transfers out of level 3	(15.0)	(17.1)	(50.3)	0.0
Fair value, measurement with unobservable inputs reconciliation, ending balance	181.9	263.3	181.9	263.3
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	0.0	(3.4)	0.0	(6.7)
Debt securities issued by foreign governments
Assets:
Fair value, measurement with unobservable inputs reconciliation, beginning balance	3.8	3.9	3.9	3.9
Purchases, sales, issuances and settlements, net	0.0	0.0	0.0	0.0
Total realized and unrealized gains (losses) included in net income	0.0	0.0	0.0	0.0
Total realized and unrealized gains (losses) included in accumulated other comprehensive income (loss)	0.1	0.0	0.0	0.0
Transfers into level 3	0.0	0.0	0.0	0.0
Transfers out of level 3	0.0	0.0	0.0	0.0
Fair value, measurement with unobservable inputs reconciliation, ending balance	3.9	3.9	3.9	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	0.0	0.0	0.0
Asset-backed securities
Assets:
Fair value, measurement with unobservable inputs reconciliation, beginning balance	17.6	67.2	24.2	60.4
Purchases, sales, issuances and settlements, net	5.9	(4.2)	(5.2)	5.8
Total realized and unrealized gains (losses) included in net income	0.0	0.0	0.0	0.0
Total realized and unrealized gains (losses) included in accumulated other comprehensive income (loss)	(0.1)	0.6	(0.6)	1.1
Transfers into level 3	0.0	0.0	0.0	0.0
Transfers out of level 3	(5.0)	(4.0)	0.0	(7.7)
Fair value, measurement with unobservable inputs reconciliation, ending balance	18.4	59.6	18.4	59.6
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	0.0	0.0
Collateralized debt obligations
Assets:
Fair value, measurement with unobservable inputs reconciliation, beginning balance	15.3	11.5		5.4
Purchases, sales, issuances and settlements, net	0.0	0.0		0.0
Total realized and unrealized gains (losses) included in net income	0.0	0.0		0.0
Total realized and unrealized gains (losses) included in accumulated other comprehensive income (loss)	0.0	0.0		0.0
Transfers into level 3	0.0	0.0		0.0
Transfers out of level 3	(15.3)	(9.0)		(2.9)
Fair value, measurement with unobservable inputs reconciliation, ending balance	0.0	2.5	0.0	2.5
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	0.0		0.0
Commercial mortgage-backed securities
Assets:
Fair value, measurement with unobservable inputs reconciliation, beginning balance		14.9		32.0
Purchases, sales, issuances and settlements, net		0.0		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		(14.9)		(32.0)
Fair value, measurement with unobservable inputs reconciliation, ending balance		0.0		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		0.0
Collateralized mortgage obligations
Assets:
Fair value, measurement with unobservable inputs reconciliation, beginning balance		0.0		0.0
Purchases, sales, issuances and settlements, net		(7.3)		(7.9)
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)		(1.4)		(1.4)
Transfers into level 3		8.9		9.5
Transfers out of level 3		0.0		0.0
Fair value, measurement with unobservable inputs reconciliation, ending balance		0.2		0.2
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		0.0		0.0
Total fixed maturities, available for sale
Assets:
Fair value, measurement with unobservable inputs reconciliation, beginning balance	236.8	386.3	258.5	360.2
Purchases, sales, issuances and settlements, net	(1.0)	(29.8)	(1.5)	(16.2)
Total realized and unrealized gains (losses) included in net income	0.1	7.2	1.3	7.8
Total realized and unrealized gains (losses) included in accumulated other comprehensive income (loss)	(0.8)	(9.7)	(3.8)	(0.8)
Transfers into level 3	4.4	20.5	0.0	21.1
Transfers out of level 3	(35.3)	(45.0)	(50.3)	(42.6)
Fair value, measurement with unobservable inputs reconciliation, ending balance	204.2	329.5	204.2	329.5
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	(3.4)	0.0	(6.7)
Equity securities
Assets:
Fair value, measurement with unobservable inputs reconciliation, beginning balance	21.4	24.3	21.2	25.2
Purchases, sales, issuances and settlements, net	(10.9)	0.0	(10.9)	(0.1)
Total realized and unrealized gains (losses) included in net income	(1.0)	0.0	(0.8)	(0.1)
Total realized and unrealized gains (losses) included in accumulated other comprehensive income (loss)	0.0	0.0	0.0	(0.4)
Transfers into level 3	0.0	0.3	0.0	0.0
Transfers out of level 3	0.0	0.0	0.0	0.0
Fair value, measurement with unobservable inputs reconciliation, ending balance	9.5	24.6	9.5	24.6
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	0.0	$ 0.0
Investments held by variable interest entities | Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3) | Investments held by variable interest entities - corporate securities
Assets:
Fair value, measurement with unobservable inputs reconciliation, beginning balance			4.9
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			(4.9)
Fair value, measurement with unobservable inputs reconciliation, ending balance	$ 0.0		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