Fair Value of Financial Instruments - Rollforward of Instruments Measured on Recurring Basis (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Financial Assets
Beginning balance	$ 399,724	$ 254,713	$ 295,178	$ 248,489
Transfers into Level 3	21,670	39,921	215,664	67,801
Transfers out of Level 3	(100,169)	(3,271)	(166,659)	(8,147)
Total gains or losses
Net investment gains (losses) included in net income related to financial assets	(265)	46	(257)	46
Net unrealized investment gains (losses) included in OCI	10,022	2,983	(10,440)	9,955
Purchases	0	0	8,765	1,566
Issuances	0	0	0	0
Sales	0	0	0	(607)
Settlements	0	0	0	0
Paydowns, maturities and distributions	(3,240)	(3,193)	(14,509)	(27,904)
Ending balance	327,742	291,199	327,742	291,199
Financial Liabilities
Beginning balance	93,619	85,961	93,733	78,700
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Total gains or losses
Net investment (gains) losses included in net income related to financial liabilities	4,406	3,661	5,330	8,366
Net unrealized investment gains (losses) included in OCI	0	0	0	0
Purchases	0	0	0	0
Issuances	1,951	2,033	5,818	7,482
Sales	0	0	0	0
Settlements	0	0	0	0
Paydowns, maturities and distributions	(1,906)	(2,557)	(6,811)	(5,450)
Ending balance	98,070	89,098	98,070	89,098
Municipal bonds
Financial Assets
Beginning balance	73,171	46,984	44,291	47,531
Transfers into Level 3	6,209	0	80,686	0
Transfers out of Level 3	(16,708)	0	(62,625)	0
Total gains or losses
Net investment gains (losses) included in net income related to financial assets	0	0	0	0
Net unrealized investment gains (losses) included in OCI	3,150	842	3,962	649
Purchases	0	0	0	0
Issuances	0	0	0	0
Sales	0	0	0	0
Settlements	0	0	0	0
Paydowns, maturities and distributions	(151)	(121)	(643)	(475)
Ending balance	65,671	47,705	65,671	47,705
Corporate bonds
Financial Assets
Beginning balance	126,292	79,222	103,952	80,742
Transfers into Level 3	6,798	18,916	39,601	24,798
Transfers out of Level 3	(12,511)	(2,822)	(26,699)	(7,698)
Total gains or losses
Net investment gains (losses) included in net income related to financial assets	0	0	0	0
Net unrealized investment gains (losses) included in OCI	551	1,744	378	6,254
Purchases	0	0	6,875	1,566
Issuances	0	0	0	0
Sales	0	0	0	0
Settlements	0	0	0	0
Paydowns, maturities and distributions	55	(387)	(2,922)	(8,989)
Ending balance	121,185	96,673	121,185	96,673
Mortgage-Backed Securities
Financial Assets
Beginning balance	200,146	128,438	146,828	120,211
Transfers into Level 3	8,663	21,004	95,377	42,938
Transfers out of Level 3	(70,950)	(449)	(77,335)	(449)
Total gains or losses
Net investment gains (losses) included in net income related to financial assets	(238)	0	(238)	0
Net unrealized investment gains (losses) included in OCI	6,321	397	(14,780)	3,052
Purchases	0	0	1,890	0
Issuances	0	0	0	0
Sales	0	0	0	(607)
Settlements	0	0	0	0
Paydowns, maturities and distributions	(3,144)	(2,685)	(10,944)	(18,440)
Ending balance	140,798	146,705	140,798	146,705
Total Fixed Maturity Securities
Financial Assets
Beginning balance	399,609	254,644	295,071	248,484
Transfers into Level 3	21,670	39,920	215,664	67,736
Transfers out of Level 3	(100,169)	(3,271)	(166,659)	(8,147)
Total gains or losses
Net investment gains (losses) included in net income related to financial assets	(238)	0	(238)	0
Net unrealized investment gains (losses) included in OCI	10,022	2,983	(10,440)	9,955
Purchases	0	0	8,765	1,566
Issuances	0	0	0	0
Sales	0	0	0	(607)
Settlements	0	0	0	0
Paydowns, maturities and distributions	(3,240)	(3,193)	(14,509)	(27,904)
Ending balance	327,654	291,083	327,654	291,083
Equity securities
Financial Assets
Beginning balance	115	69	107	5
Transfers into Level 3	0	1	0	65
Transfers out of Level 3	0	0	0	0
Total gains or losses
Net investment gains (losses) included in net income related to financial assets	(27)	46	(19)	46
Net unrealized investment 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	0	0	0
Ending balance	$ 88	$ 116	$ 88	$ 116

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

No definition available.

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

- Definition

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

+ References

No definition available.

+ Details

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

- 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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ 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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ 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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1a)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ 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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ 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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ 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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ 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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1a)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ 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

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ 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_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=us-gaap_MortgageBackedSecuritiesMember
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: