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Summary of Significant Accounting Policies - Additional Information (Detail)
 6 Months Ended
Jun. 30, 2015
USD ($)
Subsidiary
mi
Bcf
Dec. 31, 2014
USD ($)
Bcf
Jun. 30, 2014
USD ($)
Bcf
Significant Accounting Policies [Line Items]      
Length Of Pipeline | mi 86    
Cost of Removal Obligations $ 68,100,000 $ 63,800,000 $ 60,800,000
Number of Natural Gas Storage Outstanding | Bcf 2.9 2.4 2.3
Maximum      
Significant Accounting Policies [Line Items]      
Cash equivalents maturity period 3 months    
ISO-NE Obligations      
Significant Accounting Policies [Line Items]      
Cash Deposits $ 700,000 $ 6,300,000 $ 8,800,000
Natural Gas Hedging Program      
Significant Accounting Policies [Line Items]      
Cash Deposits $ 100,000 $ 0 $ 0
Utilities      
Significant Accounting Policies [Line Items]      
Number of Subsidiaries | Subsidiary 3    
Other Subsidiaries      
Significant Accounting Policies [Line Items]      
Number of Subsidiaries | Subsidiary 3