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, 2013 Bcf	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012 Bcf	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013 mi Entity Bcf	Dec. 31, 2012 Bcf	Dec. 31, 2011
Significant Accounting Policies [Line Items]
Number of Subsidiaries									3
Length Of Pipeline									86
Depreciation rate based on average depreciable property balance									3.59%	3.60%	3.43%
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
Average interest rates									1.92%	2.04%	2.28%
Cost of removal obligation	$ 57.3				$ 51.4				$ 57.3	$ 51.4
Number of Natural Gas Storage Outstanding	1.8				1.9				1.8	1.9
Intangible Assets, Purchase Adjustments									12.2
Plant
Significant Accounting Policies [Line Items]
Intangible Asset Amortization Period									10 years
Bargain Purchase Adjustment
Significant Accounting Policies [Line Items]
Intangible Asset Amortization Period									5 years
Maximum
Significant Accounting Policies [Line Items]
Cash equivalents maturity period									3 months
Quarterly Dividends
Significant Accounting Policies [Line Items]
Common stock dividend paid per share									$ 0.345	$ 0.345	$ 0.345
ISO-NE Obligations
Significant Accounting Policies [Line Items]
Cash Deposits	7.3				5.4				7.3	5.4
Natural Gas Hedging Program
Significant Accounting Policies [Line Items]
Cash Deposits	$ 0				$ 1.2				$ 0	$ 1.2
Utilities
Significant Accounting Policies [Line Items]
Number of Subsidiaries									3
Other Subsidiaries
Significant Accounting Policies [Line Items]
Number of Subsidiaries									3