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Derivative Financial Instruments - Clearinghouse Margin and Collateral (Details) - USD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Derivative contracts    
Initial margin collateral posted $ 2.1 $ 2.9
Variation margin 0.5  
Interest rate swaps    
Derivative contracts    
Financial instruments pledged as collateral 26.2 22.6
Cash pledged as collateral 2.6 4.9
Chicago Mercantile Exchange (CME)    
Derivative contracts    
Variation margin   $ 3.1
London Clearing House (LCH)    
Derivative contracts    
Variation margin $ 0.6