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Derivative Financial Instruments - Clearinghouse Margin and Collateral (Details) - USD ($)
$ in Millions
Dec. 31, 2019
Dec. 31, 2018
Derivative contracts    
Initial margin collateral posted $ 8.7 $ 2.1
Interest rate swaps    
Derivative contracts    
Financial instruments pledged as collateral 29.9 26.2
Cash pledged as collateral 13.4 2.6
Chicago Mercantile Exchange (CME)    
Derivative contracts    
Variation margin 1.0 0.5
London Clearing House (LCH)    
Derivative contracts    
Variation margin $ 62.5 $ 0.6