Quarterly report pursuant to Section 13 or 15(d)

Long-Term Debt (Narrative) (Details)

v2.4.0.8
Long-Term Debt (Narrative) (Details) (USD $)
9 Months Ended 12 Months Ended
Sep. 30, 2013
Dec. 31, 2012
Debt Instrument [Line Items]    
Senior Notes $ 191,429,000 $ 227,143,000
Applicable margin interest on credit facility, low end 1.00%  
Applicable margin interest on credit facility, high end 1.875%  
Interest Rate Spread added to LIBOR Rate 1.00%  
Applicable Margin Percentage, Interest Rate added to federal funds rate 0.50%  
Line of Credit Facility, Amount Outstanding 11,996,000 10,000,000
Letter Of Credit [Member]
   
Debt Instrument [Line Items]    
Maximum borrowing capacity 150,000,000  
Line of Credit Facility, Amount Outstanding 57,800,000 52,400,000
Sweep Program [Member]
   
Debt Instrument [Line Items]    
Maximum borrowing capacity 20,000,000  
Five Year Senior Unsecured Revolving Credit Facility [Member]
   
Debt Instrument [Line Items]    
Current borrowing capacity 200,000,000  
Maximum borrowing capacity 300,000,000  
Minimum increments under the additional borrowings 25,000,000  
Interest Rate Spread added to LIBOR Rate 1.00% 1.125%
Senior Notes [Member]
   
Debt Instrument [Line Items]    
Periodic principal payments $ 35,700,000  
Fixed interest rate, minimum 4.00%  
Fixed interest rate, maximum 5.85%  
Effective average interest rate 4.99% 5.07%
Maximum [Member]
   
Debt Instrument [Line Items]    
Interest Rate Spread added to LIBOR Rate 1.125%  
Minimum [Member]
   
Debt Instrument [Line Items]    
Interest Rate Spread added to LIBOR Rate 1.00%