Annual report pursuant to Section 13 and 15(d)

Long-Term Debt (Narrative) (Details)

v3.3.1.900
Long-Term Debt (Narrative) (Details) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Dec. 31, 2015
Dec. 31, 2015
Dec. 31, 2014
Debt Instrument [Line Items]      
Senior Notes $ 120,000 $ 120,000 $ 155,714
Long-term Line of Credit 12,317 12,317 0
Letter of Credit [Member]      
Debt Instrument [Line Items]      
Long-term Line of Credit $ 67,700 67,700 $ 63,200
Senior Notes [Member]      
Debt Instrument [Line Items]      
Periodic principal payments   $ 25,000  
Fixed interest rate, minimum   4.00%  
Fixed interest rate, maximum   5.85%  
Effective average interest rate 4.68% 4.68% 4.87%
Minimum [Member]      
Debt Instrument [Line Items]      
Interest Rate Spread added to variable rate   0.00%  
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage   0.125%  
Maximum [Member]      
Debt Instrument [Line Items]      
Interest Rate Spread added to variable rate   0.50%  
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage   0.20%  
Sweep Program [Member]      
Debt Instrument [Line Items]      
Line of Credit Facility, Capacity Available for Specific Purpose Other than for Trade Purchases $ 30,000 $ 30,000  
2015 Credit Agreement [Member]      
Debt Instrument [Line Items]      
Maximum borrowing capacity 350,000 350,000  
Current borrowing capacity $ 250,000 250,000  
Minimum increments under the additional borrowings   25,000  
Interest Rate Spread added to variable rate 1.00%    
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage 0.125%    
Letter of Credit [Member]      
Debt Instrument [Line Items]      
Line of Credit Facility, Capacity Available for Specific Purpose Other than for Trade Purchases $ 100,000 $ 100,000  
2011 Credit Agreement [Member]      
Debt Instrument [Line Items]      
Current borrowing capacity     $ 200,000
Interest Rate Spread added to variable rate   1.00% 1.00%
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage   0.175% 0.175%
London Interbank Offered Rate (LIBOR) [Member] | Minimum [Member]      
Debt Instrument [Line Items]      
Interest Rate Spread added to variable rate   1.00%  
London Interbank Offered Rate (LIBOR) [Member] | Maximum [Member]      
Debt Instrument [Line Items]      
Interest Rate Spread added to variable rate   1.50%