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Fair Value Measurements - Credit Ratings (Details)
$ in Millions
Jun. 30, 2020
USD ($)
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]  
Debt securities, held-to-maturity $ 72.3
Standard & Poor's, AA+ Rating  
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]  
Debt securities, held-to-maturity 13.5
Standard & Poor's, AA Rating  
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]  
Debt securities, held-to-maturity 10.7
Standard & Poor's, A Rating  
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]  
Debt securities, held-to-maturity 24.2
Standard & Poor's, A- Rating  
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]  
Debt securities, held-to-maturity 13.4
Standard & Poor's, BBB+ Rating  
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]  
Debt securities, held-to-maturity $ 10.5