INVESTMENTS - Schedule of Investments in Continuous Loss Position (Details) $ in Millions	Sep. 30, 2021 USD ($)
Debt Securities, Available-for-sale [Line Items]
Fair value, less than twelve months	$ 582.7
Gross unrealized losses, less than twelve months	(2.1)
Fair value, twelve months or greater	0.0
Gross unrealized losses, twelve months or greater	0.0
Fair value, total	582.7
Gross unrealized losses, total	(2.1)
U.S. government and agency securities
Debt Securities, Available-for-sale [Line Items]
Fair value, less than twelve months	57.0
Gross unrealized losses, less than twelve months	(0.1)
Fair value, twelve months or greater	0.0
Gross unrealized losses, twelve months or greater	0.0
Fair value, total	57.0
Gross unrealized losses, total	(0.1)
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Fair value, less than twelve months	190.4
Gross unrealized losses, less than twelve months	(0.8)
Fair value, twelve months or greater	0.0
Gross unrealized losses, twelve months or greater	0.0
Fair value, total	190.4
Gross unrealized losses, total	(0.8)
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Fair value, less than twelve months	335.3
Gross unrealized losses, less than twelve months	(1.2)
Fair value, twelve months or greater	0.0
Gross unrealized losses, twelve months or greater	0.0
Fair value, total	335.3
Gross unrealized losses, total	$ (1.2)

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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