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Long-Term Debt (Narrative) (Details) - USD ($)
9 Months Ended
Sep. 30, 2017
Jun. 30, 2014
Debt Instrument [Line Items]    
Standby letters of credit issued $ 35,600,000  
Revolving Credit Facility [Member]    
Debt Instrument [Line Items]    
Unused lines of credit $ 309,400,000  
2014 Credit Agreement [Member]    
Debt Instrument [Line Items]    
Revolving credit facility period, years 5 years  
2014 Credit Agreement [Member] | LIBOR [Member]    
Debt Instrument [Line Items]    
Basis spread on variable interest rate 1.13%  
2014 Credit Agreement [Member] | Revolving Credit Facility [Member]    
Debt Instrument [Line Items]    
Face amount of debt   $ 350,000,000
2014 Credit Agreement [Member] | Term Notes [Member]    
Debt Instrument [Line Items]    
Face amount of debt   $ 100,000,000