Unpaid Claims and Claim Expenses - Reconciliation of Net Incurred and Paid Claims (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Gross reserves, end of year	$ 438,754	$ 442,854
Property and Casualty segment
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Liabilities for claims and claim adjustment expenses, net of reinsurance	241,182
Reinsurance recoverables, including reinsurance premium paid	112,881
Unallocated claims adjustment expenses	18,090
Total other than short duration and unallocated claims adjustment expenses	84,691
Property and Casualty segment \| Homeowners
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Liabilities for claims and claim adjustment expenses, net of reinsurance	55,533
Reinsurance recoverables, including reinsurance premium paid	6,386
Property and Casualty segment \| Automobile liability
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Liabilities for claims and claim adjustment expenses, net of reinsurance	179,749
Reinsurance recoverables, including reinsurance premium paid	99,717
Property and Casualty segment \| Automobile physical damage
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Liabilities for claims and claim adjustment expenses, net of reinsurance	2,711
Property and Casualty segment \| Other short duration lines
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Liabilities for claims and claim adjustment expenses, net of reinsurance	3,189
Reinsurance recoverables, including reinsurance premium paid	6,778
Property and Casualty segment \| Insurance lines other than short duration
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Insurance lines other than short duration	$ 66,601

Liability For Other Than Short Duration And Unallocated Claims Adjustment Expenses

No definition available.

Name:	hmn_LiabilityForOtherThanShortDurationAndUnallocatedClaimsAdjustmentExpenses
Namespace Prefix:	hmn_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The amount needed to reflect the estimated ultimate cost of settling claims relating to insured events that have occurred on or before the balance sheet date, whether or not reported to the insurer at that date.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 50
-Paragraph 3
-Subparagraph (cc)
-URI http://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437

+ Details

Name:	us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount, after valuation allowance, recoverable under reinsurance contracts for losses reported to the ceding insurer but not yet paid and amounts expected for incurred losses and settlement expenses, which have not yet been reported to the ceding insurer.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 310
-Section 45
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=99404803&loc=d3e21412-158489

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 20
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=99397103&loc=d3e6824-158387

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 50
-Paragraph 3
-Subparagraph (cc)
-URI http://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437

+ Details

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

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

- Definition

Undiscounted amount, after reinsurance, of the liability for unpaid claims and allocated claim adjustment expense for short-duration insurance contracts. Excludes unallocated claim adjustment expense.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 50
-Paragraph 4C
-URI http://asc.fasb.org/extlink&oid=116884468&loc=SL65671315-158438

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 50
-Paragraph 4B
-URI http://asc.fasb.org/extlink&oid=116884468&loc=SL65671311-158438

+ Details

Name:	us-gaap_ShortdurationInsuranceContractsLiabilityForUnpaidClaimsAndAllocatedClaimAdjustmentExpenseNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of reconciling item for the reconciliation of the liability in the disclosure of claims development (in excess of) less than the liability for unpaid claims and claims adjustment expense for short-duration insurance contracts, classified as other.

+ References

+ Details

Name:	us-gaap_ShortdurationInsuranceContractsLiabilityForUnpaidClaimsAndClaimAdjustmentExpenseOtherReconcilingItem
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of liability for unallocated claim adjustment expense for short-duration insurance contracts.

+ References

+ Details

Name:	us-gaap_ShortdurationInsuranceContractsLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseAccumulatedUnallocatedClaimAdjustmentExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	srt_ProductOrServiceAxis=us-gaap_PropertyLiabilityAndCasualtyInsuranceSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_GuaranteedInsuranceContractTypeOfBenefitAxis=hmn_HomeownersMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_GuaranteedInsuranceContractTypeOfBenefitAxis=hmn_AutoLiabilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_GuaranteedInsuranceContractTypeOfBenefitAxis=hmn_AutoPhysicalDamageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_GuaranteedInsuranceContractTypeOfBenefitAxis=hmn_OtherShortDurationLinesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_GuaranteedInsuranceContractTypeOfBenefitAxis=hmn_OtherThanShortDurationLineMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: