Property and Casualty Unpaid Claims and Claim Expenses - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
Insurance Loss Reserves [Abstract]
Favorable development of total reserves for property and casualty claims occurring in prior years	$ 300	$ 1,000

The reduction in the estimate of the amount needed to settle claims relating to property and casualty insured events that have occurred in prior years.

No definition available.

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