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Derivatives - Interest Rate Risk (Narratives) (Details) - Derivatives designated as hedging instruments - USD ($)
$ in Millions
Sep. 30, 2018
Aug. 31, 2017
March 2022    
Derivatives    
Notional amount of interest rate swaps $ 300.0 $ 300.0
Derivative liability fair value   $ 0.7
Interest rate swaps    
Derivatives    
Deferred pre-tax losses to be reclassified during next 12 months 3.6  
Notional amount of interest rate swaps $ 500.0  
Weighted average effective fixed interest rate on interest rate swaps (as a percent) 1.80%