INVESTMENTS - SCHEDULE OF AMORTIZED COST, GROSS UNREALIZED GAINS AND LOSSES, ESTIMATED FAIR VALUE, AND ALLOWANCE FOR CREDIT LOSSES (Details) - USD ($) $ in Millions	Sep. 30, 2020	Jun. 30, 2020	Jan. 01, 2020	Dec. 31, 2019	Jan. 01, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 19,783.3			$ 19,179.5
Allowance for credit losses	(7.6)	$ (10.8)		(2.1)
Estimated fair value	22,702.9		$ 21,293.1	21,295.2	$ 18,443.7
Corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost	11,769.3			11,403.5
Gross unrealized gains	2,239.8			1,544.1
Gross unrealized losses	(31.3)			(12.3)
Allowance for credit losses	(7.2)	(10.0)		(2.1)
Estimated fair value	13,970.6			12,935.3
Other-than-temporary impairments included in accumulated other comprehensive income				0.0
United States Treasury securities and obligations of United States government corporations and agencies
Debt Securities, Available-for-sale [Line Items]
Amortized cost	162.3			161.4
Gross unrealized gains	79.5			43.3
Gross unrealized losses	0.0			(0.1)
Allowance for credit losses	0.0
Estimated fair value	241.8			204.6
Other-than-temporary impairments included in accumulated other comprehensive income				0.0
States and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized cost	2,282.7			2,002.1
Gross unrealized gains	352.7			246.1
Gross unrealized losses	(6.3)			(1.5)
Allowance for credit losses	(0.4)	(0.5)		0.0
Estimated fair value	2,628.7			2,246.7
Other-than-temporary impairments included in accumulated other comprehensive income				0.0
Foreign governments
Debt Securities, Available-for-sale [Line Items]
Amortized cost	85.6			82.6
Gross unrealized gains	18.7			13.0
Gross unrealized losses	0.0			0.0
Allowance for credit losses	0.0	0.0		0.0
Estimated fair value	104.3			95.6
Other-than-temporary impairments included in accumulated other comprehensive income				0.0
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost	1,093.0			1,352.9
Gross unrealized gains	39.1			36.8
Gross unrealized losses	(14.7)			(1.8)
Allowance for credit losses	0.0	(0.3)		0.0
Estimated fair value	1,117.4			1,387.9
Other-than-temporary impairments included in accumulated other comprehensive income				0.0
Agency residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost	60.9			89.2
Gross unrealized gains	6.7			5.8
Gross unrealized losses	0.0			0.0
Allowance for credit losses	0.0
Estimated fair value	67.6			95.0
Other-than-temporary impairments included in accumulated other comprehensive income				0.0
Non-agency residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost	1,988.9			1,871.0
Gross unrealized gains	177.7			172.3
Gross unrealized losses	(5.6)			(1.0)
Allowance for credit losses	0.0	$ 0.0		0.0
Estimated fair value	2,161.0			2,042.3
Other-than-temporary impairments included in accumulated other comprehensive income				(0.3)
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost	1,870.7			1,812.7
Gross unrealized gains	96.8			75.3
Gross unrealized losses	(16.1)			(1.0)
Allowance for credit losses	0.0
Estimated fair value	1,951.4			1,887.0
Other-than-temporary impairments included in accumulated other comprehensive income				0.0
Collateralized loan obligations
Debt Securities, Available-for-sale [Line Items]
Amortized cost	469.9			404.1
Gross unrealized gains	0.1			0.1
Gross unrealized losses	(9.9)			(3.4)
Allowance for credit losses	0.0
Estimated fair value	460.1			400.8
Other-than-temporary impairments included in accumulated other comprehensive income				0.0
Total fixed maturities, available for sale
Debt Securities, Available-for-sale [Line Items]
Amortized cost	19,783.3			19,179.5
Gross unrealized gains	3,011.1			2,136.8
Gross unrealized losses	(83.9)			(21.1)
Allowance for credit losses	(7.6)
Estimated fair value	$ 22,702.9			21,295.2
Other-than-temporary impairments included in accumulated other comprehensive income				$ (0.3)