Summary of Significant Accounting Policies - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014 mi Bcf	Dec. 31, 2013 Bcf	Mar. 31, 2013 Bcf
Significant Accounting Policies [Line Items]
Length Of Pipeline	86
Cost of removal obligation	$ 59.2	$ 57.3	$ 53.0
Number of Natural Gas Storage Outstanding	1.1	1.8	1.7
Maximum
Significant Accounting Policies [Line Items]
Cash equivalents maturity period	3 months
ISO-NE Obligations
Significant Accounting Policies [Line Items]
Cash Deposits	9.8	7.3	5.6
Natural Gas Hedging Program
Significant Accounting Policies [Line Items]
Cash Deposits	$ 0.1	$ 0	$ 0
Utilities
Significant Accounting Policies [Line Items]
Number of Subsidiaries	3
Other Subsidiaries
Significant Accounting Policies [Line Items]
Number of Subsidiaries	3