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Financial Instruments (Estimated Fair Values of Financial Instruments) (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Carrying Amount [Member] | Level 1 [Member]    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Cash and cash equivalents $ 120,457 $ 82,584
Carrying Amount [Member] | Level 2 [Member]    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Long-term debt, including current portion [1] (576,013) (586,756)
Carrying Amount [Member] | Level 2 [Member] | Interest rate contracts [Member]    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Derivative assets 1,169 1,395
Derivative liabilities related to interest rate swaps   (91)
Carrying Amount [Member] | Level 2 [Member] | Term Loans [Member]    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Long-term debt, including current portion (343,500) (349,500)
Carrying Amount [Member] | Level 2 [Member] | Senior notes [Member]    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Long-term debt, including current portion (149,528) (149,271)
Carrying Amount [Member] | Level 2 [Member] | Revenue bonds [Member]    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Long-term debt, including current portion (65,735) (65,735)
Carrying Amount [Member] | Level 2 [Member] | Medium-term notes [Member]    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Long-term debt, including current portion (17,250) (22,250)
Carrying Amount [Member] | Level 3 [Member]    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Company owned life insurance (COLI) 1,996 70
Fair Value [Member] | Level 1 [Member]    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Cash and cash equivalents 120,457 82,584
Fair Value [Member] | Level 2 [Member]    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Long-term debt, including current portion [1] (588,479) (601,290)
Fair Value [Member] | Level 2 [Member] | Interest rate contracts [Member]    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Derivative assets 1,169 1,395
Derivative liabilities related to interest rate swaps   (91)
Fair Value [Member] | Level 2 [Member] | Term Loans [Member]    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Long-term debt, including current portion (345,222) (350,909)
Fair Value [Member] | Level 2 [Member] | Senior notes [Member]    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Long-term debt, including current portion (161,063) (164,250)
Fair Value [Member] | Level 2 [Member] | Revenue bonds [Member]    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Long-term debt, including current portion (63,967) (62,205)
Fair Value [Member] | Level 2 [Member] | Medium-term notes [Member]    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Long-term debt, including current portion (18,227) (23,926)
Fair Value [Member] | Level 3 [Member]    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Company owned life insurance (COLI) $ 1,996 $ 70
[1] The carrying amount of long-term debt includes principal and unamortized discounts.