Fair Value of Financial Instruments - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Fair Value of Financial Instruments (Textual) [Abstract]
Net realized investment gains (losses) included in net income related to financial assets	$ 0	$ 161	$ 243	$ 1,875
Level 3
Fair Value of Financial Instruments (Textual) [Abstract]
Percentage of invested assets in total investment portfolio Level 3 recurring	3.20%		3.20%
Net realized investment losses	$ (2,205)	$ (2,587)	$ (1,308)	$ (6,133)

Fair value of financial instruments.

No definition available.

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

- Definition

Percentage of invested assets in the total investment portfolio in Level 3 carried at fair value on a recurring basis.

+ References

No definition available.

+ Details

Name:	hmn_PercentageOfInvestedAssetsInTotalInvestmentPortfolioLevel3Recurring
Namespace Prefix:	hmn_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Net Increase or Decrease in the fair value of the embedded derivative or group of embedded derivatives included in earnings in the period.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 15
-URI http://asc.fasb.org/subtopic&trid=2229187

+ Details

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

- Definition

Amount of gain (loss) recognized in the income statement for 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)(1)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

+ Details

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

- Details

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