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Summary of Contractual Obligations for Long-term Debt (Detail) $ in Millions | Dec. 31, 2021 USD ($)
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Contractual Obligation Fiscal Year Maturity Schedule [Line Items] |
Total | $ 538.1
2022 | 59.7
2023 | 51.8
2024 | 40.0
2025 | 38.6
2026 | 38.2
2027 & Beyond | 309.8
Long Term Debt Comprising Principal And Interest [Member] |
Contractual Obligation Fiscal Year Maturity Schedule [Line Items] |
Total | 870.1
2022 | 32.9
2023 | 30.8
2024 | 30.3
2025 | 27.9
2026 | 60.6
2027 & Beyond | 687.6
Long-Term Debt [Member] | Long Term Debt Comprising Principal And Interest [Member] |
Contractual Obligation Fiscal Year Maturity Schedule [Line Items] |
Total | 509.6
2022 | 8.4
2023 | 6.9
2024 | 6.9
2025 | 5.0
2026 | 38.0
2027 & Beyond | 444.4
Interest on Long-Term Debt [Member] | Long Term Debt Comprising Principal And Interest [Member] |
Contractual Obligation Fiscal Year Maturity Schedule [Line Items] |
Total | 360.5
2022 | 24.5
2023 | 23.9
2024 | 23.4
2025 | 22.9
2026 | 22.6
2027 & Beyond | $ 243.2

X
- Definition Amount of contractual obligation, including but not limited to, long-term debt, capital lease obligations, operating lease obligations, purchase obligations, and other commitments. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of contractual obligation to be paid after fifth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach). + References No definition available. + Details
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Namespace Prefix: | us-gaap_
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Period Type: | instant

X
- Definition Amount of contractual obligation to be paid in fifth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach). + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
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X
- Definition Amount of contractual obligation to be paid in fourth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach). + References No definition available. + Details
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Namespace Prefix: | us-gaap_
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Period Type: | instant

X
- Definition Amount of contractual obligation to be paid in next fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach). + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of contractual obligation to be paid in second fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach). + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of contractual obligation to be paid in third fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach). + References No definition available. + Details
Name: | us-gaap_ContractualObligationDueInThirdYear
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- References No definition available. + Details
Name: | utl_ContractualObligationFiscalYearMaturityScheduleLineItems
Namespace Prefix: | utl_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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

X
- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |