Summary of Significant Accounting Policies - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2014 mi Bcf	Dec. 31, 2013 Bcf	Jun. 30, 2013 Bcf
Significant Accounting Policies [Line Items]
Length Of Pipeline	86
Cost of removal obligation	$ 60.8	$ 57.3	$ 54.7
Construction Work in Progress	34.3	24.6	40.4
Number of Natural Gas Storage Outstanding	2.3	1.8	1.8
Maximum
Significant Accounting Policies [Line Items]
Cash equivalents maturity period	3 months
Customer information system
Significant Accounting Policies [Line Items]
Construction Work in Progress	5.0
ISO-NE Obligations
Significant Accounting Policies [Line Items]
Cash Deposits	8.8	7.3	4.8
Natural Gas Hedging Program
Significant Accounting Policies [Line Items]
Cash Deposits	$ 0	$ 0	$ 0.7
Utilities
Significant Accounting Policies [Line Items]
Number of Subsidiaries	3
Other Subsidiaries
Significant Accounting Policies [Line Items]
Number of Subsidiaries	3