INVESTMENTS - Schedule of Investments in Continuous Loss Position (Details) $ in Millions	Mar. 31, 2021 USD ($)
Debt Securities, Available-for-sale [Line Items]
Fair value, less than twelve months	$ 296.1
Gross unrealized losses, less than twelve months	(1.8)
Fair value, twelve months or greater	0.0
Gross unrealized losses, twelve months or greater	0.0
Fair value, total	296.1
Gross unrealized losses, total	(1.8)
U.S. government and agency securities
Debt Securities, Available-for-sale [Line Items]
Fair value, less than twelve months	23.1
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	23.1
Gross unrealized losses, total	(0.1)
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Fair value, less than twelve months	58.6
Gross unrealized losses, less than twelve months	(0.2)
Fair value, twelve months or greater	0.0
Gross unrealized losses, twelve months or greater	0.0
Fair value, total	58.6
Gross unrealized losses, total	(0.2)
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Fair value, less than twelve months	214.4
Gross unrealized losses, less than twelve months	(1.5)
Fair value, twelve months or greater	0.0
Gross unrealized losses, twelve months or greater	0.0
Fair value, total	214.4
Gross unrealized losses, total	$ (1.5)

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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