INVESTMENTS IN VARIABLE INTEREST ENTITIES - BALANCE SHEET ITEMS (Details) - USD ($) $ in Millions	Sep. 30, 2020	Jan. 01, 2020	Dec. 31, 2019	Jan. 01, 2019
Assets:
Investments held by variable interest entities	$ 1,172.6	$ 1,178.7	$ 1,188.6
Cash and cash equivalents held by variable interest entities	51.0		74.7
Accrued investment income	214.4		205.9
Income tax assets, net	322.0	437.5	432.6	$ 630.9
Other assets	472.3		476.0
Total assets	34,569.2	$ 33,613.1	33,630.9	$ 31,436.7
Liabilities:
Other liabilities	855.8		750.2
Borrowings related to variable interest entities	1,152.0		1,152.5
Total liabilities	29,485.3		28,953.9
Variable Interest Entity, Primary Beneficiary
Assets:
Investments held by variable interest entities	1,172.6		1,188.6
Notes receivable of VIEs held by subsidiaries	(113.8)		(113.8)
Cash and cash equivalents held by variable interest entities	51.0		74.7
Accrued investment income	1.8		1.7
Income tax assets, net	16.1		8.0
Other assets	4.8		1.4
Total assets	1,132.5		1,160.6
Liabilities:
Other liabilities	40.9		38.4
Borrowings related to variable interest entities	1,152.0		1,152.5
Notes payable of VIEs held by subsidiaries	0.0		0.0
Total liabilities	1,192.9		1,190.9
Variable Interest Entity, Primary Beneficiary | VIEs
Assets:
Investments held by variable interest entities	1,172.6		1,188.6
Notes receivable of VIEs held by subsidiaries	0.0		0.0
Cash and cash equivalents held by variable interest entities	51.0		74.7
Accrued investment income	1.8		1.7
Income tax assets, net	16.1		8.0
Other assets	5.7		2.8
Total assets	1,247.2		1,275.8
Liabilities:
Other liabilities	45.7		42.8
Borrowings related to variable interest entities	1,152.0		1,152.5
Notes payable of VIEs held by subsidiaries	126.1		126.1
Total liabilities	1,323.8		1,321.4
Variable Interest Entity, Primary Beneficiary | Eliminations
Assets:
Investments held by variable interest entities	0.0		0.0
Notes receivable of VIEs held by subsidiaries	(113.8)		(113.8)
Cash and cash equivalents held by variable interest entities	0.0		0.0
Accrued investment income	0.0		0.0
Income tax assets, net	0.0		0.0
Other assets	(0.9)		(1.4)
Total assets	(114.7)		(115.2)
Liabilities:
Other liabilities	(4.8)		(4.4)
Borrowings related to variable interest entities	0.0		0.0
Notes payable of VIEs held by subsidiaries	(126.1)		(126.1)
Total liabilities	$ (130.9)		$ (130.5)