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Derivative Financial Instruments - Clearinghouse Margin and Collateral (Details) - USD ($)
$ in Millions
Mar. 31, 2019
Dec. 31, 2018
Derivative contracts    
Initial margin collateral posted $ 29.2 $ 2.1
Interest rate swaps    
Derivative contracts    
Financial instruments pledged as collateral 0.0 26.2
Cash pledged as collateral 30.7 2.6
Chicago Mercantile Exchange (CME)    
Derivative contracts    
Variation margin 0.8 0.5
London Clearing House (LCH)    
Derivative contracts    
Variation margin $ 26.3 $ 0.6