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Debt (Schedule of Long-Term Debt) (Details) - USD ($)
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2009
Debt Instrument [Line Items]        
Long-term Debt, Gross $ 607,585,000 $ 630,085,000    
Less debt issuance costs (3,037,000) (3,675,000)    
Less unamortized discounts (1,248,000) (1,535,000)    
Long-term Debt 603,881,000 625,668,000    
Less current portion of long-term debt (5,007,000) (22,870,000)    
Long term debt [1] 598,874,000 602,798,000    
Term loans, variable rate 1.98% to 2.23% due 2019 through 2021 and fixed rate 2.95% to 4.64% due 2017 through 2024        
Debt Instrument [Line Items]        
Long-term Debt, Gross $ 322,000,000 322,000,000    
Term loans, variable rate 1.98% to 2.23% due 2019 through 2021 and fixed rate 2.95% to 4.64% due 2017 through 2024 | Minimum [Member]        
Debt Instrument [Line Items]        
Interest Rate 1.98%      
Term loans, variable rate 1.98% to 2.23% due 2019 through 2021 and fixed rate 2.95% to 4.64% due 2017 through 2024 | Maximum [Member]        
Debt Instrument [Line Items]        
Interest Rate 2.23%      
Senior notes, 7.50%, due 2019        
Debt Instrument [Line Items]        
Interest Rate 7.50%     7.50%
Long-term Debt, Gross $ 150,000,000 150,000,000    
Revenue bonds, 5.90% to 6.95%, due 2024 and 2026        
Debt Instrument [Line Items]        
Long-term Debt, Gross $ 108,335,000 108,335,000    
Revenue bonds, 5.90% to 6.95%, due 2024 and 2026 | Minimum [Member]        
Debt Instrument [Line Items]        
Interest Rate 5.90%      
Revenue bonds, 5.90% to 6.95%, due 2024 and 2026 | Maximum [Member]        
Debt Instrument [Line Items]        
Interest Rate 6.95%      
Debentures, 6.95%, due 2015        
Debt Instrument [Line Items]        
Interest Rate 6.95%      
Long-term Debt, Gross $ 0 22,500,000    
Medium-term notes, 8.75% to 8.89%, due 2016 through 2022        
Debt Instrument [Line Items]        
Long-term Debt, Gross $ 27,250,000 27,250,000    
Medium-term notes, 8.75% to 8.89%, due 2016 through 2022 | Minimum [Member]        
Debt Instrument [Line Items]        
Interest Rate 8.75%      
Medium-term notes, 8.75% to 8.89%, due 2016 through 2022 | Maximum [Member]        
Debt Instrument [Line Items]        
Interest Rate 8.89%      
Interest rate swaps        
Debt Instrument [Line Items]        
Long-term Debt, Gross $ 581,000 $ 793,000    
Debt Instrument, Redemption, Period One [Member] | Term loans, variable rate 1.98% to 2.23% due 2019 through 2021 and fixed rate 2.95% to 4.64% due 2017 through 2024        
Debt Instrument [Line Items]        
Interest Rate     2.95%  
[1] Debt issuance costs in 2014 were reclassified to conform with the 2015 presentation.