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Debt (Details) (USD $)
0 Months Ended 9 Months Ended 0 Months Ended
Oct. 31, 2013
FFB
Sep. 30, 2013
FFB
Apr. 23, 2013
Pollution control revenue bonds
Mar. 02, 2013
Pollution control revenue bonds
Debt        
Debt issued $ 13,217,000 $ 662,880,000 $ 212,760,000  
Refund of pollution control revenue bonds       $ 212,760,000
Interest rate (as a percent)     2.40%