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Regulatory Matters - Additional Information (Detail) (USD $) In Millions, unless otherwise specified			12 Months Ended					12 Months Ended	1 Months Ended			3 Months Ended				1 Months Ended					1 Months Ended			12 Months Ended		1 Months Ended		12 Months Ended		1 Months Ended	12 Months Ended				1 Months Ended					1 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 Minimum	Dec. 31, 2012 Maximum	Dec. 31, 2012 New Hampshire Division Environmental Restoration Costs	Dec. 31, 2012 Maine Division Environmental Restoration Costs	Dec. 31, 2012 Other Restructuring	Dec. 31, 2012 Environmental Matters	Oct. 31, 2012 Fitchburg Gas and Electric Light Company	Aug. 01, 2011 Fitchburg Gas and Electric Light Company Y	Dec. 16, 2011 Fitchburg Gas and Electric Light Company	Mar. 31, 2011 Fitchburg Gas and Electric Light Company Environmental Restoration Costs	Dec. 31, 2012 Fitchburg Gas and Electric Light Company Environmental Restoration Costs	Dec. 31, 2012 Fitchburg Gas and Electric Light Company Environmental Clean Up Costs	Dec. 31, 2011 Fitchburg Gas and Electric Light Company Environmental Clean Up Costs	Dec. 16, 2011 Fitchburg Gas and Electric Light Company Tropical Storm Irene	Dec. 16, 2011 Fitchburg Gas and Electric Light Company Severe Snow Storm	Nov. 02, 2011 Fitchburg Gas and Electric Light Company Natural Gas Court	Dec. 31, 2009 Fitchburg Gas and Electric Light Company Natural Gas Court	Nov. 02, 2011 Fitchburg Gas and Electric Light Company Natural Gas Supreme Court	Aug. 01, 2011 Fitchburg Gas and Electric Light Company Electric Distribution	Aug. 01, 2011 Fitchburg Gas and Electric Light Company Gas Distribution	Jun. 29, 2012 Granite State Gas Transmission Inc Gas Transportation and Storage	Dec. 31, 2012 Granite State Gas Transmission Inc Gas Transportation and Storage	Dec. 31, 2012 Granite State Gas Transmission Inc Asset Recoverable Gas Costs	Oct. 31, 2012 Unitil Energy Systems Inc	Aug. 31, 2011 Unitil Energy Systems Inc	Dec. 31, 2012 Unitil Energy Systems Inc	Feb. 28, 2011 Unitil Energy Systems Inc	Dec. 16, 2011 Unitil Energy Systems Inc Storm Costs	Dec. 31, 2012 Unitil Energy Systems Inc Minimum	Dec. 31, 2012 Unitil Energy Systems Inc Maximum	Dec. 31, 2012 Unitil Energy Systems Inc Range One	Dec. 31, 2012 Unitil Energy Systems Inc Range Two	Apr. 26, 2011 Unitil Energy Systems Inc Year One Policy	Apr. 26, 2011 Unitil Energy Systems Inc Year Two Policy	Apr. 24, 2012 Northern Utilities Inc	Dec. 31, 2012 Northern Utilities Inc	Dec. 31, 2011 Northern Utilities Inc	Nov. 29, 2011 Northern Utilities Inc Maine Operations	Nov. 29, 2011 Northern Utilities Inc Cast Iron Pipe Replacement Capital Spending
Regulatory Assets [Line Items]
Regulatory Assets , Remaining Balance	$ 24.3
Accrued Revenue	58.1	54.2					13.3
Cost recovery period, years			1 year	2 years																										5 years
Increase in annual revenue																					3.30	3.70													5.20	5.00	3.70			7.80	0.85
Return of Equity										9.20%																							7.00%	10.00%
Deferred Emergency Storm Restoration Cost										11.4
Amortized and Recovered Cost, Years										7
Additional Cost to Transmission Plant																							2.4	11.4
Amount being Recovered																									4.0
Annual revenue increase in rate adjustments																							0.3
Amendment effective date																																			2010-07-01	2011-05-01	May 1, 2012			January 1, 2012	May 1, 2012
Estimated increases for Capital Improvement																															1.5	2.0
Settlement agreement date																												May 01, 2016									Apr. 24, 2012
Adjustment to recover the increased spending for vegetation management program and reliability enhancement program																												1.5
Recovery of damage of electrical system									1.1																	2.6
Storm expenditures																1.5	3.3
Portion of storm cost expenditures that the Company has requested deferral of and recovery of through rate adjustments											4.3
Recovery of damage of electrical system																														4.4
Interest rate allowed by regulators on the unrecovered amount of storm costs which have been approved for recovery through rate adjustments																														4.52%
Refund of Cost																		4.5		0.2
Regulatory Liabilities																		4.7	4.9
Customer's Overpayment Amount																											1.8
Amount Reimbursed to Customer																											1.8
Interest on Reimbursement																											0.3
Non-Recurring Charge on Distribution																											0.4
Request for recovery of under-billed customers																													1.4
Income (loss) incurred in excess of recorded amount								0
Amortization period for environmental costs					7 years	5 years							7 years
Settlement insurance												2.0
Environmental Obligations														12.0	12.0
Environmental Obligations	$ 14.8	$ 14.7																																				$ 2.8	$ 2.7

X

- Definition

Total costs accrued as of the balance sheet date for environmental loss contingencies.

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Carrying value as of the balance sheet date of obligations incurred and payable, pertaining to costs that are statutory in nature, are incurred on contractual obligations, or accumulate over time and for which invoices have not yet been received or will not be rendered. Examples include taxes, interest, rent and utilities. Used to reflect the current portion of the liabilities (due within one year or within the normal operating cycle if longer).

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Carrying amount as of the balance sheet date of the portion of the reserved funds established by a utility from customer payments to cover the repair costs of damages from major storms, which repairs are expected to be incurred beyond one year or the normal operating cycle, if longer.

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The increase (decrease) during the reporting period in carrying amount of capitalized electric costs of a regulated entity that are expected to be recoverable through rate adjustments within one year or the normal operating cycle, if longer.

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The increase (decrease) during the reporting period in unrecovered fuel costs for a regulated utility company.

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Expenses charged against earnings in the period of an event that is infrequent in occurrence or unusual in nature.

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Gas costs of a regulated entity that are refundable through future rate adjustments. Such costs are a form of a regulatory liabilities and are expected to be refunded in less than one year through rate adjustments.

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The amount for the individual regulatory liability as itemized in a table of regulatory liabilities as of the end of the period.

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Repayment received or receivable for expenses incurred on behalf of a client or customer, other than those reimbursements received by landlords from tenants.

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Remaining balance of regulatory assets for which no return on investment during recovery is provided. In some cases, a regulator may permit an enterprise to include a cost that would be charged to expense by an unregulated enterprise as an allowable cost over a period of time by amortizing that cost for rate-making purposes, but the regulator does not include the unrecovered amount in the rate base.

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Remaining recovery period of regulatory assets for which no return on investment during recovery period is provided, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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A noncash adjustment to the provision established for storm damage by an oil, gas, or utility company.

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Unbilled amounts due for services rendered or to be rendered, actions taken or to be taken, or a promise to refrain from taking certain actions in accordance with the terms of a legally binding agreement between the entity and, at a minimum, one other party. An example would be amounts associated with contracts or programs where the recognized revenue for performance thereunder exceeds the amounts billed under the terms thereof as of the date of the balance sheet.

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Represents the interest rate allowed by regulators on the unrecovered amount of storm costs which have been approved for recovery through rate adjustments.

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Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Represents an amount for which the company has requested approval from regulators for to recover from customers for underbilling.

+ References

No definition available.

+ Details

Name:	utl_RequestForRecoveryOfUnderbilling
Namespace Prefix:	utl_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

Settlement Dates

+ References

No definition available.

+ Details

Name:	utl_SettlementDates
Namespace Prefix:	utl_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Expenditures related to specific storms.

+ References

No definition available.

+ Details

Name:	utl_StormExpenditures
Namespace Prefix:	utl_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration