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INVESTMENTS - Schedule of Cost and Fair Value of Investments in Debt Securities, By Contractual Maturity (Details) - USD ($)
$ in Millions
Sep. 30, 2017
Dec. 31, 2016
Cost    
Due in 1 year or less $ 617.5  
Due after 1 year through 5 years 0.0  
Instruments not due at a single maturity date 4.5  
Cost 622.0 $ 238.3
Fair Value    
Due in 1 year or less 617.5  
Due after 1 year through 5 years 0.0  
Instruments not due at a single maturity date 4.5  
Fair Value 622.0 238.2
Cost    
Due in 1 year or less 155.0  
Due after 1 year through 5 years 435.2  
Instruments not due at a single maturity date 111.4  
Cost 701.6 603.0
Fair Value    
Due in 1 year or less 155.0  
Due after 1 year through 5 years 435.2  
Instruments not due at a single maturity date 111.4  
Fair Value $ 701.6 $ 600.9