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Long-Term Debt and Financing Obligations (Details) - USD ($)
$ in Thousands
Sep. 30, 2019
May 22, 2019
Apr. 01, 2019
Feb. 01, 2019
Dec. 31, 2018
Debt Instrument [Line Items]          
Line of Credit Facility, Current Borrowing Capacity $ 350,000        
Potential borrowing capacity of indenture $ 200,000        
7.25% 2009 Series A Refunding Bonds, due 2040          
Debt Instrument [Line Items]          
Debt Instrument, Interest Rate, Stated Percentage   3.60%      
7.25% 2009 Series B Refunding Bonds, due 2040          
Debt Instrument [Line Items]          
Debt Instrument, Interest Rate, Stated Percentage   3.60%      
Pollution Control Bonds          
Debt Instrument [Line Items]          
Debt Instrument, Face Amount         $ 159,800
Long-term Debt, Subject for Redemption         $ 100,600
Pollution Control Bonds | 7.25% 2009 Series A Refunding Bonds, due 2040          
Debt Instrument [Line Items]          
Debt Instrument, Face Amount   $ 63,500      
Debt Instrument, Interest Rate, Stated Percentage   3.60%   7.25%  
Debt Instrument, Repurchased Face Amount       $ 63,500  
Pollution Control Bonds | 7.25% 2009 Series B Refunding Bonds, due 2040          
Debt Instrument [Line Items]          
Debt Instrument, Face Amount   $ 37,100      
Debt Instrument, Interest Rate, Stated Percentage   3.60% 7.25%    
Debt Instrument, Repurchased Face Amount     $ 37,100