Consolidated Statements of Cash Flows (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Operating Activities:
Net Income	$ 24.7us-gaap_NetIncomeLoss	$ 21.6us-gaap_NetIncomeLoss	$ 18.2us-gaap_NetIncomeLoss
Adjustments to Reconcile Net Income to Cash Provided by Operating Activities:
Depreciation and Amortization	42.1	38.5	36.0
Deferred Taxes Provision	14.4us-gaap_DeferredIncomeTaxExpenseBenefit	12.3us-gaap_DeferredIncomeTaxExpenseBenefit	10.8us-gaap_DeferredIncomeTaxExpenseBenefit
Changes in Working Capital Items:
Accounts Receivable	(8.5)	(4.5)	(2.6)
Accrued Revenue	8.1	6.8	(0.7)
Regulatory Liabilities	(1.0)	2.9	(4.4)
Taxes Refundable / Payable	(0.1)	(0.5)	(0.3)
Exchange Gas Receivable	(4.2)	(1.4)	4.1
Accounts Payable	6.1	5.4	5.4
Other Changes in Working Capital Items	6.6	(2.9)	(0.4)
Deferred Regulatory and Other Charges	(1.6)	15.0	5.4
Other, net	(2.6)	3.1	(4.8)
Cash Provided by Operating Activities	84.0	96.3	66.7
Investing Activities:
Property, Plant and Equipment Additions	(92.6)	(89.5)	(68.5)
Cash (Used In) Investing Activities	(92.6)	(89.5)	(68.5)
Financing Activities:
Proceeds from (Repayment of) Short-Term Debt, net	(30.9)	10.8	(38.5)
Issuance of Long-Term Debt	50.0
Repayment of Long-Term Debt	(4.4)us-gaap_RepaymentsOfLongTermDebt	(0.5)us-gaap_RepaymentsOfLongTermDebt	(0.5)us-gaap_RepaymentsOfLongTermDebt
Increase / (Decrease) in Capital Lease Obligations	6.5	(0.7)	(0.9)
Net Increase (Decrease) in Exchange Gas Financing	4.4	1.2	(3.8)
Dividends Paid	(19.2)us-gaap_PaymentsOfDividends	(19.1)us-gaap_PaymentsOfDividends	(17.2)us-gaap_PaymentsOfDividends
Retirement of Preferred Stock			(1.8)
Proceeds from Issuance of Common Stock	1.2	1.1	66.8
Cash (Used In) Provided by Financing Activities	7.6	(7.2)	4.1
Net Increase (Decrease) in Cash	(1.0)	(0.4)	2.3
Cash at Beginning of Year	9.4	9.8	7.5
Cash at End of Year	8.4	9.4	9.8
Supplemental Information:
Interest Paid	20.8us-gaap_InterestPaidNet	20.8us-gaap_InterestPaidNet	21.2us-gaap_InterestPaidNet
Income Taxes Paid	1.2us-gaap_IncomeTaxesPaidNet	0.8us-gaap_IncomeTaxesPaidNet	0.7us-gaap_IncomeTaxesPaidNet
Capital Expenditures Included in Accounts Payable	$ 0.3	$ 0.7	$ 1.9

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The net change during the reporting period in deferred regulatory and other charges.

+ References

No definition available.

+ Details

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

- Definition

The net change during the reporting period in exchange gas financing.

+ References

No definition available.

+ Details

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

- Definition

Value of preferred stock that has been repurchased and retired during the period. The excess of the purchase price over par value can be charged against retained earnings (once the excess is fully allocated to additional paid in capital).

+ References

No definition available.

+ Details

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

- Definition

Proceeds from (Repayments of) Debt and Capital Lease Obligations

+ References

No definition available.

+ Details

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