Amount before allocation of valuation allowances of deferred tax asset attributable to deductible alternative minimum tax credit carryforwards.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Alternative Minimum Tax
-URI http://asc.fasb.org/extlink&oid=6504908

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=65882285&loc=d3e32559-109319

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=65882285&loc=d3e32632-109319

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=65882285&loc=d3e32621-109319

+ Details

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

- Definition

A brief description of status of the tax examination, significant findings to date, and the entity's position with respect to the findings.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (d)(2)
-URI http://asc.fasb.org/extlink&oid=65882285&loc=d3e32718-109319

+ Details

Name:	us-gaap_IncomeTaxExaminationDescription
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 9
-Subparagraph (a),(b)
-URI http://asc.fasb.org/extlink&oid=65882285&loc=d3e32639-109319

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(h))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Income Tax Expense (or Benefit)
-URI http://asc.fasb.org/extlink&oid=6515339

+ Details

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

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_OperatingLossCarryforwardsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Net Operating Loss And Tax Credit Carry Forward

+ References

No definition available.

+ Details

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

- Definition

Net Operating Loss Carryforwards, Expiration Year

+ References

No definition available.

+ Details

Name:	utl_NetOperatingLossCarryforwardsExpirationYear
Namespace Prefix:	utl_
Data Type:	xbrli:gYearItemType
Balance Type:	na
Period Type:	duration

- Definition

Tax Credit Carryforward, Expiration Year

+ References

No definition available.

+ Details

Name:	utl_TaxCreditCarryforwardExpirationYear
Namespace Prefix:	utl_
Data Type:	xbrli:gYearItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_TaxCreditCarryforwardAxis=utl_UsFederalResearchAndDevelopmentTaxCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TaxPeriodAxis=us-gaap_TaxYear2014Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TaxPeriodAxis=utl_TaxYear2015Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TaxPeriodAxis=utl_TaxYear2016Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_IncomeTaxAuthorityNameAxis=us-gaap_InternalRevenueServiceIRSMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_IncomeTaxAuthorityAxis=utl_FederalMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Income Taxes - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Operating Loss Carryforwards [Line Items]
Alternative Minimum tax credit carryforwards					$ 2,100,000
Income tax benefit	$ 1,200,000	$ 1,100,000	$ 8,200,000	$ 10,000,000
Federal Research and Development Tax Credit \| Tax Year 2014
Operating Loss Carryforwards [Line Items]
Income tax benefit					(63,000)
Federal Research and Development Tax Credit \| Tax Year 2015
Operating Loss Carryforwards [Line Items]
Income tax benefit					(288,000)
Federal Research and Development Tax Credit \| Tax Year 2016
Operating Loss Carryforwards [Line Items]
Income tax benefit			$ (241,900)
Internal Revenue Service (IRS)
Operating Loss Carryforwards [Line Items]
NOL carryforward assets and Research and Development (R&D) credit carryforwards					$ 11,300,000
NOL carryforward assets expiration date					2029
R&D credits expiration year					2035
Federal
Operating Loss Carryforwards [Line Items]
Tax examination description			The Company remains subject to examination by Federal, Maine, Massachusetts, and New Hampshire tax authorities for the tax periods ended December 31, 2012; December 31, 2013; and December 31, 2014.