Quarterly report pursuant to Section 13 or 15(d)

Long-Term Debt (Narrative) (Details)

v3.7.0.1
Long-Term Debt (Narrative) (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Dec. 31, 2016
Debt Instrument [Line Items]          
Senior Notes $ 95,000   $ 95,000   $ 95,000
Interest Rate Spread added to Rate 1.00% 1.00% 1.00% 1.00%  
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage 0.125% 0.125% 0.125% 0.125%  
Letter Of Credit Fee In Percentage 1.00% 1.00% 1.00% 1.00%  
Long-term Line of Credit $ 0   $ 0   $ 9,975
Senior Notes [Member]          
Debt Instrument [Line Items]          
Effective average interest rate 4.37%   4.37%   4.37%
Tranche A [Member]          
Debt Instrument [Line Items]          
Debt Instrument, Periodic Payment, Principal $ 50,000        
Fixed interest rate 4.00%   4.00%    
Tranche B [Member]          
Debt Instrument [Line Items]          
Debt Instrument, Periodic Payment, Principal $ 45,000        
Fixed interest rate 4.79%   4.79%    
Maximum [Member]          
Debt Instrument [Line Items]          
Interest Rate Spread added to Rate 0.50%        
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage 0.20%        
Minimum [Member]          
Debt Instrument [Line Items]          
Interest Rate Spread added to Rate 0.00%        
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    
Long-term Line of Credit 71,400   71,400   $ 74,600
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]          
Line of Credit Facility, Maximum Borrowing Capacity 350,000   350,000    
Original borrowing capacity 250,000   250,000    
Line Of Credit Facility Accordion 100,000   100,000    
Accordion Feature Exercised 50,000   50,000    
Line of Credit Facility, Current Borrowing Capacity $ 300,000   $ 300,000    
London Interbank Offered Rate (LIBOR) [Member] | Maximum [Member]          
Debt Instrument [Line Items]          
Interest Rate Spread added to Rate 1.50%        
London Interbank Offered Rate (LIBOR) [Member] | Minimum [Member]          
Debt Instrument [Line Items]          
Interest Rate Spread added to Rate 1.00%