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Details on Preferred Stock (Parenthetical) (Detail) (USD $)	6 Months Ended		12 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Unitil Energy Systems Inc | Noncumulative Preferred Stock | Series 6 | Nonredeemable Preferred Stock
Preferred Units [Line Items]
Dividend rate	6.00%	6.00%	6.00%
Preferred Stock Par Value	$ 100	$ 100	$ 100
Fitchburg Gas and Electric Light Company | Series 5
Preferred Units [Line Items]
Dividend rate			5.125%
Fitchburg Gas and Electric Light Company | Series 8
Preferred Units [Line Items]
Dividend rate			8.00%
Fitchburg Gas and Electric Light Company | Cumulative Preferred Stock | Series 5 | Redeemable Preferred Stock
Preferred Units [Line Items]
Dividend rate	5.125%	5.125%	5.125%
Preferred Stock Par Value	$ 100	$ 100	$ 100
Fitchburg Gas and Electric Light Company | Cumulative Preferred Stock | Series 8 | Redeemable Preferred Stock
Preferred Units [Line Items]
Dividend rate	8.00%	8.00%	8.00%
Preferred Stock Par Value	$ 100	$ 100	$ 100

X

- Definition

The percentage rate used to calculate dividend payments on preferred stock.

+ References

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X

- Definition

Face amount or stated value per share of preferred stock nonredeemable or redeemable solely at the option of the issuer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.28)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 29

-Article 5

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