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Derivative Instruments (Narrative) (Details)
$ in Millions
1 Months Ended 6 Months Ended 12 Months Ended
Dec. 31, 2017
USD ($)
Jun. 30, 2018
USD ($)
Derivative
Dec. 31, 2017
USD ($)
Derivatives Fair Value [Line Items]      
Net losses expected to be reclassified into earnings in the next twelve months   $ 0.5  
Interest Rate Swaps [Member]      
Derivatives Fair Value [Line Items]      
Number of interest rate swap agreements | Derivative   3  
Notional amounts   $ 14.3  
Derivative original maturity date   Nov. 30, 2019  
Interest expense over the next twelve months     $ 0.2
Interest Rate Swaps [Member] | Cash Flow Hedging [Member]      
Derivatives Fair Value [Line Items]      
Number of interest rate swap agreements | Derivative   4  
Interest Rate Swaps [Member] | Senior notes [Member]      
Derivatives Fair Value [Line Items]      
Notional amounts associated with senior notes $ 50.0   $ 50.0
Derivative, termination year   2017-12  
Termination cost $ 0.4