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Summary of Significant Accounting Policies - Additional Information (Detail)
 9 Months Ended
Sep. 30, 2012
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Dec. 31, 2011
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Sep. 30, 2011
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Significant Accounting Policies [Line Items]      
Number of Operating Gas Transmission 86    
Natural Gas Outstanding under purchase contract 2,100,000,000 1,600,000,000 1,700,000,000
Other Subsidiaries
      
Significant Accounting Policies [Line Items]      
Number of Subsidiaries 3    
Utilities
      
Significant Accounting Policies [Line Items]      
Number of Subsidiaries 3