Fair Value of Financial Instruments - Rollforward of Instruments Measured on Recurring Basis (Details) - USD ($)	3 Months Ended	9 Months Ended
Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Financial Assets
Beginning balance	$ 478,800,000	$ 420,000,000.0	$ 425,300,000	$ 371,400,000
Transfers into Level 3	0	33,500,000	36,900,000	179,300,000
Transfers out of Level 3	(3,700,000)	(34,800,000)	(3,700,000)	(42,900,000)
Total gains or losses
Net gains/losses included in net income	200,000	(900,000)	200,000	(3,200,000)
Net unrealized gains/losses included in OCI	(4,900,000)	(16,600,000)	(6,700,000)	(41,600,000)
Purchases	19,200,000	16,500,000	54,600,000	21,400,000
Issuances	0	0	0	0
Sales	0	0	(7,700,000)	(2,100,000)
Settlements	0	0	0	0
Paydowns, maturities and distributions	(12,000,000.0)	(15,400,000)	(21,300,000)	(80,000,000.0)
Ending balance	477,600,000	402,300,000	477,600,000	402,300,000
Financial Liabilities
Beginning balance	83,600,000	97,800,000	91,300,000	111,400,000
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Total gains or losses
Net gains/losses included in net income	(3,200,000)	(6,300,000)	(5,600,000)	(13,900,000)
Net unrealized gains/losses included in OCI	400,000	0	(300,000)	(2,400,000)
Purchases	0	0	0	0
Issuances	2,000,000.0	2,700,000	6,300,000	4,800,000
Sales	0	0	0	0
Settlements	0	0	0	0
Paydowns, maturities and distributions	(4,500,000)	(1,400,000)	(13,400,000)	(7,100,000)
Ending balance	78,300,000	92,800,000	78,300,000	92,800,000
Level 3
Total gains or losses
Net gains/losses included in net income	200,000	(900,000)	200,000	(3,200,000)
Total gains or losses
Net gains/losses included in net income	(3,200,000)	(6,300,000)	(5,600,000)	(13,900,000)
Municipal bonds
Financial Assets
Beginning balance	63,900,000	51,000,000.0	54,400,000	60,800,000
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	(3,200,000)
Total gains or losses
Net gains/losses included in net income	0	0	0	0
Net unrealized gains/losses included in OCI	5,900,000	(5,300,000)	6,500,000	(11,600,000)
Purchases	0	200,000	9,500,000	200,000
Issuances	0	0	0	0
Sales	0	0	0	0
Settlements	0	0	0	0
Paydowns, maturities and distributions	(100,000)	7,400,000	(700,000)	7,100,000
Ending balance	69,700,000	53,300,000	69,700,000	53,300,000
Corporate bonds
Financial Assets
Beginning balance	311,500,000	270,500,000	261,300,000	210,300,000
Transfers into Level 3	0	20,500,000	36,100,000	144,000,000.0
Transfers out of Level 3	(3,700,000)	(34,800,000)	(3,700,000)	(34,900,000)
Total gains or losses
Net gains/losses included in net income	0	0	0	0
Net unrealized gains/losses included in OCI	(12,800,000)	(7,400,000)	(14,100,000)	(18,700,000)
Purchases	17,500,000	8,400,000	43,200,000	13,300,000
Issuances	0	0	0	0
Sales	0	0	(7,700,000)	0
Settlements	0	0	0	0
Paydowns, maturities and distributions	(7,300,000)	(17,400,000)	(9,900,000)	(74,200,000)
Ending balance	305,200,000	239,800,000	305,200,000	239,800,000
Mortgage Backed and Other Asset- Backed Securities
Financial Assets
Beginning balance	101,400,000	97,100,000	107,600,000	98,900,000
Transfers into Level 3	0	12,200,000	800,000	34,500,000
Transfers out of Level 3	0	0	0	(4,800,000)
Total gains or losses
Net gains/losses included in net income	0	(800,000)	0	(3,100,000)
Net unrealized gains/losses included in OCI	2,000,000.0	(3,900,000)	900,000	(11,300,000)
Purchases	1,700,000	7,900,000	1,900,000	7,900,000
Issuances	0	0		0
Sales	0	0		(2,100,000)
Settlements	0	0		0
Paydowns, maturities and distributions	(4,600,000)	(5,300,000)	(10,700,000)	(12,800,000)
Ending balance	100,500,000	107,200,000	100,500,000	107,200,000
Total Fixed Maturity Securities
Financial Assets
Beginning balance	476,800,000	418,600,000	423,300,000	370,000,000.0
Transfers into Level 3	0	32,700,000	36,900,000	178,500,000
Transfers out of Level 3	(3,700,000)	(34,800,000)	(3,700,000)	(42,900,000)
Total gains or losses
Net gains/losses included in net income	0	(800,000)	0	(3,100,000)
Net unrealized gains/losses included in OCI	(4,900,000)	(16,600,000)	(6,700,000)	(41,600,000)
Purchases	19,200,000	16,500,000	54,600,000	21,400,000
Issuances	0	0	0	0
Sales	0	0	(7,700,000)	(2,100,000)
Settlements	0	0	0	0
Paydowns, maturities and distributions	(12,000,000.0)	(15,300,000)	(21,300,000)	(79,900,000)
Ending balance	475,400,000	400,300,000	475,400,000	400,300,000
Equity securities
Financial Assets
Beginning balance	2,000,000.0	1,400,000	2,000,000.0	1,400,000
Transfers into Level 3	0	800,000	0	800,000
Transfers out of Level 3	0	0	0	0
Total gains or losses
Net gains/losses included in net income	200,000	(100,000)	200,000	(100,000)
Net unrealized gains/losses included in OCI	0	0	0	0
Purchases	0	0	0	0
Issuances	0	0	0	0
Sales	0	0	0	0
Settlements	0	0	0	0
Paydowns, maturities and distributions	0	(100,000)	0	(100,000)
Ending balance	$ 2,200,000	$ 2,000,000.0	$ 2,200,000	$ 2,000,000.0

Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Paydowns And Maturities

No definition available.

Name:	hmn_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPaydownsAndMaturities
Namespace Prefix:	hmn_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Gain Or Loss Net [Abstract]

+ References

No definition available.

+ Details

Name:	hmn_GainOrLossNetAbstract
Namespace Prefix:	hmn_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Gain Or Losses Net [Abstract]

+ References

No definition available.

+ Details

Name:	hmn_GainOrLossesNetAbstract
Namespace Prefix:	hmn_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Fair value measurement with unobservable inputs reconciliation recurring basis asset paydowns and maturities.

+ References

No definition available.

+ Details

Name:	hmn_PaydownsMaturitiesAndDistributions
Namespace Prefix:	hmn_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- References

No definition available.

+ Details

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

- Definition

Amount of transfers of financial instrument classified as a liability into level 3 of the fair value hierarchy.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (c)(3)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482106/820-10-50-2

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationLiabilityTransfersIntoLevel3
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of transfers of financial instrument classified as a liability out of level 3 of the fair value hierarchy.

+ References

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationLiabilityTransfersOutOfLevel3
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of gain (loss) recognized in income from asset measured at fair value on recurring basis using unobservable input (level 3).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (c)(1)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482106/820-10-50-2

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of gain (loss) recognized in other comprehensive income (OCI) from asset measured at fair value on recurring basis using unobservable input (level 3).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (c)(1a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482106/820-10-50-2

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of issuances of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482106/820-10-50-2

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetIssues
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of purchases of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of sale of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of settlement of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of transfer of financial instrument classified as an asset into level 3 of the fair value hierarchy.

+ References

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of transfers of financial instrument classified as an asset out of level 3 of the fair value hierarchy.

+ References

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Fair value of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (c)
-SubTopic 10
-Topic 820
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482106/820-10-50-2

+ Details

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

- Definition

Amount of gain (loss) recognized in income from liability measured at fair value on recurring basis using unobservable input (level 3).

+ References

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of gain (loss) recognized in other comprehensive income (OCI) from liability measured at fair value on recurring basis using unobservable input (level 3).

+ References

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInOtherComprehensiveIncome
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of issuances of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityIssues
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of purchases of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchases
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of sales of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySales
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of settlements of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Fair value of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

+ Details

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

- References

No definition available.

+ Details

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

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_MunicipalBondsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CorporateDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=hmn_MortgageBackedAndOtherAssetBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_DebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_EquitySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: