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Derivatives - Interest Rate Risk (Details) - Derivatives designated as hedging instruments: - USD ($)
$ in Millions
Sep. 30, 2017
Aug. 31, 2017
Interest rate swaps    
Notional Disclosures    
Notional amount of interest rate swaps $ 500.0  
May 2019    
Notional Disclosures    
Notional amount of interest rate swaps 100.0  
May 2020    
Notional Disclosures    
Notional amount of interest rate swaps 100.0  
March 2022    
Notional Disclosures    
Notional amount of interest rate swaps $ 300.0 $ 300.0