XML 129 R99.htm IDEA: XBRL DOCUMENT
v3.25.0.1

Employee Postretirement Benefits - Schedule of Weighted-Average Assumptions Used to Determine Net Periodic Benefit Cost (Details) | 12 Months Ended
--- | ---
Dec. 31, 2024 | Dec. 31, 2023 | Dec. 31, 2022
Pension Benefits (Pension Plan) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Discount Rate | 5.57% | 5.51% | 3.03%
Long-Term Rate of Return on Plan Assets | 7.00% | 7.00% | 6.30%
Pension Benefits (Pension Plan) | Participants up to Age 39 |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Long-Term Rate of Compensation Increase | 4.50% | 4.50% | 4.50%
Pension Benefits (Pension Plan) | Participants Ages 40 to 49 |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Long-Term Rate of Compensation Increase | 4.00% | 3.50% | 3.50%
Pension Benefits (Pension Plan) | Participants Age 50 and Older |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Long-Term Rate of Compensation Increase | 3.38% | 2.75% | 2.75%
Pension Benefits (ESSRP) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Discount Rate | 5.53% | 5.51% | 2.93%
Long-Term Rate of Compensation Increase | 3.00% | 3.00% | 3.00%
Postretirement Benefits |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Discount Rate | 5.53% | 5.52% | 3.01%

X
- Definition Weighted average rate for present value of future retirement benefits cash flows, used to determine net periodic benefit cost of defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (k)(1) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480506/715-20-50-1 + Details
Name: | us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate
Namespace Prefix: | us-gaap_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Weighted average rate of return on plan assets, reflecting average rate of earnings expected on existing plan assets and expected contributions, used to determine net periodic benefit cost of defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 8 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480506/715-20-50-8 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (k)(3) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480506/715-20-50-1 + Details
Name: | us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets
Namespace Prefix: | us-gaap_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Weighted average rate of compensation increase used to determine net periodic benefit cost of defined benefit plan. Plan includes, but is not limited to, pay-related defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (k)(2) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480506/715-20-50-1 + Details
Name: | us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostRateOfCompensationIncrease
Namespace Prefix: | us-gaap_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | duration

X
- 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_DefinedBenefitPlanDisclosureLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_RetirementPlanTypeAxis=us-gaap_PensionPlansDefinedBenefitMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_TitleOfIndividualAxis=ottr_ParticipantsUpToAge39Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_TitleOfIndividualAxis=ottr_ParticipantsAge40To49Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_TitleOfIndividualAxis=ottr_ParticipantsAge50AndOlderMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RetirementPlanTypeAxis=ottr_ExecutiveSurvivorAndSupplementalRetirementPlanMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RetirementPlanTypeAxis=us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |