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Activity in Company's Allowance for Doubtful Accounts (Detail) (USD $) In Millions, unless otherwise specified | 12 Months Ended |
--- | --- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011
Accounts and Other Receivables [Line Items] |  |  |  |
Balance at Beginning of Period | $ 1.6 us-gaap_AllowanceForDoubtfulAccountsReceivable | $ 1.9 us-gaap_AllowanceForDoubtfulAccountsReceivable | $ 2.3 us-gaap_AllowanceForDoubtfulAccountsReceivable |
Provision | 6.0 us-gaap_ProvisionForDoubtfulAccounts | 4.6 us-gaap_ProvisionForDoubtfulAccounts | 3.6 us-gaap_ProvisionForDoubtfulAccounts |
Recoveries | 0.6 us-gaap_AllowanceForDoubtfulAccountsReceivableRecoveries | 0.4 us-gaap_AllowanceForDoubtfulAccountsReceivableRecoveries | 0.6 us-gaap_AllowanceForDoubtfulAccountsReceivableRecoveries |
Accounts Written Off | 6.4 us-gaap_AllowanceForDoubtfulAccountsReceivableWriteOffs | 5.3 us-gaap_AllowanceForDoubtfulAccountsReceivableWriteOffs | 4.6 us-gaap_AllowanceForDoubtfulAccountsReceivableWriteOffs |
Balance at End of Period | 1.8 us-gaap_AllowanceForDoubtfulAccountsReceivable | 1.6 us-gaap_AllowanceForDoubtfulAccountsReceivable | 1.9 us-gaap_AllowanceForDoubtfulAccountsReceivable |
Electric |  |  |  |
Accounts and Other Receivables [Line Items] |  |  |  |
Balance at Beginning of Period | 1.3 us-gaap_AllowanceForDoubtfulAccountsReceivable / invest_InvestmentHoldingAxis = utl_ElectricMember | 1.1 us-gaap_AllowanceForDoubtfulAccountsReceivable / invest_InvestmentHoldingAxis = utl_ElectricMember | 1.7 us-gaap_AllowanceForDoubtfulAccountsReceivable / invest_InvestmentHoldingAxis = utl_ElectricMember |
Provision | 2.9 us-gaap_ProvisionForDoubtfulAccounts / invest_InvestmentHoldingAxis = utl_ElectricMember | 2.6 us-gaap_ProvisionForDoubtfulAccounts / invest_InvestmentHoldingAxis = utl_ElectricMember | 1.4 us-gaap_ProvisionForDoubtfulAccounts / invest_InvestmentHoldingAxis = utl_ElectricMember |
Recoveries | 0.3 us-gaap_AllowanceForDoubtfulAccountsReceivableRecoveries / invest_InvestmentHoldingAxis = utl_ElectricMember | 0.2 us-gaap_AllowanceForDoubtfulAccountsReceivableRecoveries / invest_InvestmentHoldingAxis = utl_ElectricMember | 0.3 us-gaap_AllowanceForDoubtfulAccountsReceivableRecoveries / invest_InvestmentHoldingAxis = utl_ElectricMember |
Accounts Written Off | 3.2 us-gaap_AllowanceForDoubtfulAccountsReceivableWriteOffs / invest_InvestmentHoldingAxis = utl_ElectricMember | 2.6 us-gaap_AllowanceForDoubtfulAccountsReceivableWriteOffs / invest_InvestmentHoldingAxis = utl_ElectricMember | 2.3 us-gaap_AllowanceForDoubtfulAccountsReceivableWriteOffs / invest_InvestmentHoldingAxis = utl_ElectricMember |
Balance at End of Period | 1.3 us-gaap_AllowanceForDoubtfulAccountsReceivable / invest_InvestmentHoldingAxis = utl_ElectricMember | 1.3 us-gaap_AllowanceForDoubtfulAccountsReceivable / invest_InvestmentHoldingAxis = utl_ElectricMember | 1.1 us-gaap_AllowanceForDoubtfulAccountsReceivable / invest_InvestmentHoldingAxis = utl_ElectricMember |
Gas |  |  |  |
Accounts and Other Receivables [Line Items] |  |  |  |
Balance at Beginning of Period | 0.2 us-gaap_AllowanceForDoubtfulAccountsReceivable / invest_InvestmentHoldingAxis = utl_GasMember | 0.7 us-gaap_AllowanceForDoubtfulAccountsReceivable / invest_InvestmentHoldingAxis = utl_GasMember | 0.5 us-gaap_AllowanceForDoubtfulAccountsReceivable / invest_InvestmentHoldingAxis = utl_GasMember |
Provision | 3.1 us-gaap_ProvisionForDoubtfulAccounts / invest_InvestmentHoldingAxis = utl_GasMember | 2.0 us-gaap_ProvisionForDoubtfulAccounts / invest_InvestmentHoldingAxis = utl_GasMember | 2.2 us-gaap_ProvisionForDoubtfulAccounts / invest_InvestmentHoldingAxis = utl_GasMember |
Recoveries | 0.3 us-gaap_AllowanceForDoubtfulAccountsReceivableRecoveries / invest_InvestmentHoldingAxis = utl_GasMember | 0.2 us-gaap_AllowanceForDoubtfulAccountsReceivableRecoveries / invest_InvestmentHoldingAxis = utl_GasMember | 0.3 us-gaap_AllowanceForDoubtfulAccountsReceivableRecoveries / invest_InvestmentHoldingAxis = utl_GasMember |
Accounts Written Off | 3.2 us-gaap_AllowanceForDoubtfulAccountsReceivableWriteOffs / invest_InvestmentHoldingAxis = utl_GasMember | 2.7 us-gaap_AllowanceForDoubtfulAccountsReceivableWriteOffs / invest_InvestmentHoldingAxis = utl_GasMember | 2.3 us-gaap_AllowanceForDoubtfulAccountsReceivableWriteOffs / invest_InvestmentHoldingAxis = utl_GasMember |
Balance at End of Period | 0.4 us-gaap_AllowanceForDoubtfulAccountsReceivable / invest_InvestmentHoldingAxis = utl_GasMember | 0.2 us-gaap_AllowanceForDoubtfulAccountsReceivable / invest_InvestmentHoldingAxis = utl_GasMember | 0.7 us-gaap_AllowanceForDoubtfulAccountsReceivable / invest_InvestmentHoldingAxis = utl_GasMember |
Other |  |  |  |
Accounts and Other Receivables [Line Items] |  |  |  |
Balance at Beginning of Period |  |  |  | 0.1 us-gaap_AllowanceForDoubtfulAccountsReceivable / invest_InvestmentHoldingAxis = utl_OthersMember
Balance at End of Period | $ 0.1 us-gaap_AllowanceForDoubtfulAccountsReceivable / invest_InvestmentHoldingAxis = utl_OthersMember | $ 0.1 us-gaap_AllowanceForDoubtfulAccountsReceivable / invest_InvestmentHoldingAxis = utl_OthersMember | $ 0.1 us-gaap_AllowanceForDoubtfulAccountsReceivable / invest_InvestmentHoldingAxis = utl_OthersMember | $ 0.1 us-gaap_AllowanceForDoubtfulAccountsReceivable / invest_InvestmentHoldingAxis = utl_OthersMember

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- Definition For an unclassified balance sheet, a valuation allowance for receivables due a company that are expected to be uncollectible. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 4 -Article 5 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5074-111524 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.4) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Amount of recoveries of receivables doubtful of collection that were previously charged off. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Amount of direct write-downs of accounts receivable charged against the allowance. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5074-111524 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.4) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 954 -SubTopic 310 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=35755819&loc=SL9156395-115594 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Amount of expense related to write-down of receivables to the amount expected to be collected. Includes, but is not limited to, accounts receivable and notes receivable. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.5) -URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 5 -Article 5 + Details
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- Details
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Balance Type: | na
Period Type: | duration

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- Details
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Namespace Prefix: |
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Balance Type: |
Period Type: |

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- Details
Name: | invest_InvestmentHoldingAxis=utl_GasMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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