Investments - Summary of Fair Value and Amortized Costs (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost/Cost	$ 7,419,491	$ 7,302,950
Amortized Cost/Cost		116,320
Fair Value	7,529,139	7,724,075
Federal National Mortgage Association Certificates and Obligations (FNMA)
Unrealized gains and losses on fixed maturities and equity securities
Fair Value	408,130	361,955
Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC)
Unrealized gains and losses on fixed maturities and equity securities
Fair Value	408,858	400,001
Government National Mortgage Association Certificates and Obligations (GNMA)
Unrealized gains and losses on fixed maturities and equity securities
Fair Value	96,018	104,168
Total Fixed Maturity Securities
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost/Cost	7,419,491	7,302,950
Unrealized Investment Gains	233,212	443,633
Unrealized Investment Losses	123,564	22,508
Fair Value	7,529,139	7,724,075
Mortgage-backed securities
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost/Cost	758,301	669,297
Unrealized Investment Gains	15,744	30,460
Unrealized Investment Losses	23,209	3,032
Fair Value	750,836	696,725
Other, including U.S. Treasury securities
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost/Cost	802,508	714,613
Unrealized Investment Gains	12,528	26,311
Unrealized Investment Losses	32,436	5,516
Fair Value	782,600	735,408
Municipal bonds
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost/Cost	1,783,711	1,711,581
Unrealized Investment Gains	119,671	184,107
Unrealized Investment Losses	19,708	2,435
Fair Value	1,883,674	1,893,253
Foreign government bonds
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost/Cost	92,860	96,780
Unrealized Investment Gains	2,787	5,958
Unrealized Investment Losses	644	0
Fair Value	95,003	102,738
Corporate bonds
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost/Cost	2,290,452	2,409,426
Unrealized Investment Gains	74,409	173,862
Unrealized Investment Losses	29,365	4,334
Fair Value	2,335,496	2,578,954
Other mortgage-backed securities
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost/Cost	1,691,659	1,701,253
Unrealized Investment Gains	8,073	22,935
Unrealized Investment Losses	18,202	7,191
Fair Value	$ 1,681,530	1,716,997
Equity securities
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost/Cost		116,320
Unrealized Investment Gains		19,425
Unrealized Investment Losses		279
Fair Value		$ 135,466

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

+ Details

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

- Definition

This item represents the cost of equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments for other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References

No definition available.

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

+ Details

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

- Definition

Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

No definition available.

+ Details

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

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

+ Details

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

- Definition

Amount of equity securities categorized neither as held-to-maturity nor as trading.

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember
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_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=hmn_OtherIncludingUsTreasurySecuritiesMember
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_ForeignGovernmentDebtMember
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_MortgageBackedSecuritiesOtherMember
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: