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Allowance for Doubtful Accounts - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Financing Receivable, Impaired [Line Items]      
Provision for Bad Debt $ 4.6 $ 3.6 $ 4.3
Energy Commodity
     
Financing Receivable, Impaired [Line Items]      
Provision for Bad Debt $ 1.4 $ 1.9 $ 1.8