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Long-Term Debt, Schedule of Long-Term Debt (Details) - USD ($)
 12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Debt Instrument [Line Items]    
Long-term Debt $ 778,834,000 $ 716,700,000
Revolving Credit Facility Due April 30, 2020 [Member]    
Debt Instrument [Line Items]    
Credit facility, amount outstanding $ 278,834,000 0
Credit facility, expiration date Apr. 30, 2020  
Long term debt, face amount $ 550,000,000  
Revolving Credit Facility Due November 9, 2015 [Member]    
Debt Instrument [Line Items]    
Credit facility, amount outstanding $ 0 116,700,000
Credit facility, expiration date Nov. 09, 2015  
Long term debt, face amount $ 325,000,000  
Senior Notes Series A Due February 27, 2020 [Member]    
Debt Instrument [Line Items]    
Credit facility, amount outstanding $ 150,000,000 150,000,000
Credit facility, expiration date Feb. 27, 2020  
Long term debt, face amount $ 150,000,000  
Senior Notes Series B Due February 27, 2023 [Member]    
Debt Instrument [Line Items]    
Credit facility, amount outstanding $ 350,000,000 350,000,000
Credit facility, expiration date Feb. 27, 2023  
Long term debt, face amount $ 350,000,000  
Credit Line Due June 30, 2017 [Member]    
Debt Instrument [Line Items]    
Credit facility, amount outstanding $ 0 0
Credit facility, expiration date Jun. 30, 2017  
Long term debt, face amount $ 10,000,000  
Term Loan Due July 1, 2016 [Member]    
Debt Instrument [Line Items]    
Credit facility, amount outstanding $ 0 100,000,000
Credit facility, expiration date Jul. 01, 2016  
Long term debt, face amount $ 540,000,000  
Revolving Credit Facility [Member]    
Debt Instrument [Line Items]    
Long term debt, face amount   $ 325,000,000