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Derivative Financial Instruments - Counterparty Credit Risk and Credit-Risk Related Contingent Features (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Credit-risk-related contingent features      
Aggregate fair value of derivative instruments in a net liability position $ 4.0 $ 0.8  
Collateral posted for derivatives in a net liability position 4.7 0.5  
Interest rate swaps      
Derivative contracts      
Collateral thresholds for derivative agreements with credit risk related contingent features 0.5    
Counterparty credit risk adjustments $ 0.1 $ 0.1 $ 0.1