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INVESTMENTS - Credit Ratings (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Mar. 31, 2021
Jun. 30, 2020
Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]      
Amortized Cost $ 137.2   $ 218.4
Fair Value $ 139.3   224.0
AFS securities | Credit rating      
Debt Securities, Available-for-sale [Line Items]      
Concentration percentage 100.00% 100.00%  
AAA      
Debt Securities, Available-for-sale [Line Items]      
Amortized Cost $ 73.2   116.5
Fair Value $ 74.0   118.7
AAA | AFS securities | Credit rating      
Debt Securities, Available-for-sale [Line Items]      
Concentration percentage 53.10% 53.00%  
AA+, AA, AA-, A-1      
Debt Securities, Available-for-sale [Line Items]      
Amortized Cost $ 17.0   22.7
Fair Value $ 17.3   23.3
AA+, AA, AA-, A-1 | AFS securities | Credit rating      
Debt Securities, Available-for-sale [Line Items]      
Concentration percentage 12.50% 10.40%  
A+, A, A-      
Debt Securities, Available-for-sale [Line Items]      
Amortized Cost $ 36.4   57.5
Fair Value $ 37.1   59.4
A+, A, A- | AFS securities | Credit rating      
Debt Securities, Available-for-sale [Line Items]      
Concentration percentage 26.60% 26.50%  
BBB+, BBB, BBB-      
Debt Securities, Available-for-sale [Line Items]      
Amortized Cost $ 10.6   21.7
Fair Value $ 10.9   $ 22.6
BBB+, BBB, BBB- | AFS securities | Credit rating      
Debt Securities, Available-for-sale [Line Items]      
Concentration percentage 7.80% 10.10%