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Allowance for Doubtful Accounts Included in Accounts Receivable Net (Detail) - USD ($) $ in Millions | Mar. 31, 2023 | Dec. 31, 2022 | Mar. 31, 2022
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Allowance for Doubtful Accounts Included in Accounts Receivable Net Detail [Abstract] |  |  |
Allowance for Doubtful Accounts | $ 3.1 | $ 2.6 | $ 3.4

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- Definition Amount of allowance for credit loss on accounts receivable. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 50 -Paragraph 13 -Subparagraph (a) -URI https://asc.fasb.org/extlink&oid=124255953&loc=SL82919249-210447 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 4 -URI https://asc.fasb.org/extlink&oid=123577603&loc=d3e5074-111524 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 50 -Paragraph 13 -Subparagraph (f) -URI https://asc.fasb.org/extlink&oid=124255953&loc=SL82919249-210447 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 45 -Paragraph 1 -URI https://asc.fasb.org/extlink&oid=124255206&loc=SL82895884-210446 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(4)) -URI https://asc.fasb.org/extlink&oid=120391452&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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- References No definition available. + Details
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Namespace Prefix: | utl_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration