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Summary of Significant Accounting Policies - Additional Information (Detail) (USD $)
In Millions, except Per Share data, unless otherwise specified
 3 Months Ended 12 Months Ended
Dec. 31, 2012
Bcf
Sep. 30, 2012
Jun. 30, 2012
Mar. 31, 2012
Dec. 31, 2011
Bcf
Sep. 30, 2011
Jun. 30, 2011
Mar. 31, 2011
Dec. 31, 2012
Entity
mi
Bcf
Dec. 31, 2011
Bcf
Dec. 31, 2010
Significant Accounting Policies [Line Items]                      
Number of Subsidiaries                 3    
Length of pipeline                 86    
Average interest rates                 2.04% 2.28% 2.25%
Cost of removal obligation $ 51.4       $ 46.5       $ 51.4 $ 46.5  
Intangible Asset Amortization Period 6 years               6 years    
Intangible Assets, Purchase Adjustments                 14.7    
Number of Natural Gas Storage Outstanding 1,900,000,000       1,600,000,000       1,900,000,000 1,600,000,000  
Depreciation rate based on average depreciable property balance                 3.60% 3.43% 3.99%
Current Deferred Income Taxes 13.4       9.0       13.4 9.0  
Common stock dividend paid per share $ 0.345 $ 0.345 $ 0.345 $ 0.345 $ 0.345 $ 0.345 $ 0.345 $ 0.345 $ 1.38 $ 1.38 $ 1.38
Plant
                      
Significant Accounting Policies [Line Items]                      
Intangible Asset Amortization Period 10 years               10 years    
Maximum
                      
Significant Accounting Policies [Line Items]                      
Cash equivalents investment maturity period                 3 months    
Quarterly Dividends
                      
Significant Accounting Policies [Line Items]                      
Common stock dividend paid per share                 $ 0.345 $ 0.345 $ 0.345
Prior to December 31, 2012
                      
Significant Accounting Policies [Line Items]                      
Current Deferred Income Taxes 9.0               9.0    
ISO-NE Obligations
                      
Significant Accounting Policies [Line Items]                      
Cash Deposits 5.4       4.6       5.4 4.6  
Natural Gas Hedging Program
                      
Significant Accounting Policies [Line Items]                      
Cash Deposits $ 1.2       $ 2.6       $ 1.2 $ 2.6  
Utilities
                      
Significant Accounting Policies [Line Items]                      
Number of Subsidiaries                 3    
Other Subsidiaries
                      
Significant Accounting Policies [Line Items]                      
Number of Subsidiaries                 3