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INVESTMENTS - Schedule of Investments in Continuous Loss Position (Details) - USD ($)
$ in Millions
Jun. 30, 2018
Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]    
Fair value, less than twelve months $ 354.0 $ 385.3
Gross unrealized losses, less than twelve months (4.6) (1.7)
Fair value, twelve months or greater 105.0 119.5
Gross unrealized losses, twelve months or greater (1.6) (1.2)
Fair value, total 459.0 504.8
Gross unrealized losses, total (6.2) (2.9)
Commercial paper    
Debt Securities, Available-for-sale [Line Items]    
Fair value, less than twelve months 24.6 2.4
Gross unrealized losses, less than twelve months 0.0 0.0
Fair value, twelve months or greater 0.0 0.0
Gross unrealized losses, twelve months or greater 0.0 0.0
Fair value, total 24.6 2.4
Gross unrealized losses, total 0.0 0.0
U.S. government and agency securities    
Debt Securities, Available-for-sale [Line Items]    
Fair value, less than twelve months 31.3 31.5
Gross unrealized losses, less than twelve months (0.4) (0.2)
Fair value, twelve months or greater 37.0 37.1
Gross unrealized losses, twelve months or greater (0.5) (0.5)
Fair value, total 68.3 68.6
Gross unrealized losses, total (0.9) (0.7)
Foreign government bonds    
Debt Securities, Available-for-sale [Line Items]    
Fair value, less than twelve months 3.0 3.0
Gross unrealized losses, less than twelve months 0.0 0.0
Fair value, twelve months or greater 0.0 0.0
Gross unrealized losses, twelve months or greater 0.0 0.0
Fair value, total 3.0 3.0
Gross unrealized losses, total 0.0 0.0
Asset-backed securities    
Debt Securities, Available-for-sale [Line Items]    
Fair value, less than twelve months 73.7 90.8
Gross unrealized losses, less than twelve months (0.8) (0.3)
Fair value, twelve months or greater 16.1 23.2
Gross unrealized losses, twelve months or greater (0.2) (0.1)
Fair value, total 89.8 114.0
Gross unrealized losses, total (1.0) (0.4)
Corporate debt securities    
Debt Securities, Available-for-sale [Line Items]    
Fair value, less than twelve months 217.1 253.3
Gross unrealized losses, less than twelve months (3.4) (1.2)
Fair value, twelve months or greater 51.9 59.2
Gross unrealized losses, twelve months or greater (0.9) (0.6)
Fair value, total 269.0 312.5
Gross unrealized losses, total (4.3) (1.8)
Municipal securities    
Debt Securities, Available-for-sale [Line Items]    
Fair value, less than twelve months 4.3 4.3
Gross unrealized losses, less than twelve months 0.0 0.0
Fair value, twelve months or greater 0.0 0.0
Gross unrealized losses, twelve months or greater 0.0 0.0
Fair value, total 4.3 4.3
Gross unrealized losses, total $ 0.0 $ 0.0