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Long-Term Debt (Narrative) (Details) - Jun. 30, 2015 - USD ($)
Total
Debt Instrument [Line Items]  
Standby letters of credit issued $ 36,600,000
Revolving Credit Facility [Member]  
Debt Instrument [Line Items]  
Unused lines of credit $ 248,400,000
2014 Credit Agreement [Member]  
Debt Instrument [Line Items]  
Debt instrument term 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] | Medium-term Notes [Member]  
Debt Instrument [Line Items]  
Face amount of debt $ 100,000,000