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Fair Value of Financial Instruments Fair Value Disclosures (Details) - USD ($)
$ in Thousands
Mar. 31, 2017
Dec. 31, 2016
Mar. 31, 2016
Dec. 31, 2015
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]        
Cash and Cash Equivalents $ 61,266 $ 60,475 $ 426,646 $ 72,574
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]        
Long-term Debt 2,622,358 2,724,672    
Reported Value Measurement [Member]        
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]        
Cash and Cash Equivalents 61,266 60,475    
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]        
Long-term Debt 2,647,857 2,752,371    
Portion at Fair Value Measurement [Member]        
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]        
Cash and Cash Equivalents 61,266 60,475    
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]        
Long-term Debt $ 2,628,205 $ 2,717,582