Exchange Gas Receivable (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2013
Receivables [Line Items]
Total Exchange Gas Receivable	$ 1.3	$ 10.8	$ 1.7
Northern Utilities Inc
Receivables [Line Items]
Total Exchange Gas Receivable	1.0	9.8	1.5
Fitchburg Gas and Electric Light Company
Receivables [Line Items]
Total Exchange Gas Receivable	$ 0.3	$ 1.0	$ 0.2

The carrying amount of other receivables, net, due within one year of the balance sheet date (or one operating cycle, if longer) from third parties or arising from transactions not separately disclosed.

No definition available.

Name:	us-gaap_OtherReceivablesNetCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant