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Commitments and Contingencies (Details) (USD $)
 48 Months Ended 12 Months Ended 12 Months Ended
Dec. 31, 2004
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2013
Average Quarterly Utilization of Facility is 20% or More [Member]
Dec. 31, 2013
Average Quarterly Utilization of Facility is Less than 20% [Member]
Dec. 31, 2013
Revolving Credit Facility [Member]
One-Month London Interbank Offer Rate, Reset Daily [Member]
Dec. 31, 2013
Revolving Credit Facility [Member]
One-Month, Two-Month, Three-Month, Six-Month, or Twelve-Month London Interbank Offer Rate [Member]
Dec. 31, 2013
Unsecured Debt [Member]
Revolving Credit Facility [Member]
Dec. 31, 2013
Unsecured Debt [Member]
Letter of Credit Subfacility [Member]
Dec. 31, 2013
Swing Line Loans [Member]
One-Month London Interbank Offer Rate, Reset Daily [Member]
Line of Credit Facility [Line Items]                    
Line of credit facility, maximum borrowing capacity               $ 125,000,000 $ 40,000,000  
Face amount of outstanding undrawn letters of credit   34,415,000                
Credit facility, amount outstanding   0                
Interest percentage over LIBOR rate           0.875% 0.875%     0.875%
Percentage fee paid for unused portion of line of credit       0.10% 0.125%          
Commission fee on amount available to be drawn on letter of credit subfacility                 0.875%  
Issuance fee as percent of face amount                 0.075%  
Percentage purchase of outstanding IRB bonds 100.00%                  
Investment in and outstanding debt for industrial revenue bond   $ 3,200,000 $ 3,200,000