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Derivative Financial Instruments - Clearinghouse Margin and Collateral (Details) - USD ($)
$ in Millions
Sep. 30, 2018
Dec. 31, 2017
Derivative contracts    
Initial margin collateral posted $ 1.4 $ 2.9
Interest rate swaps    
Derivative contracts    
Financial instruments pledged as collateral 23.5 22.6
Cash pledged as collateral 2.0 4.9
Chicago Mercantile Exchange (CME)    
Derivative contracts    
Variation margin 0.6 $ 3.1
London Clearing House (LCH)    
Derivative contracts    
Variation margin $ 25.7