Other Comprehensive Income (Loss) (Reclassifications out of Accumulated Other Comprehensive Income) (Parenthetical) (Details) - Defined Benefit Pension and Postretirement Plans [Member]	3 Months Ended	9 Months Ended
Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Comprehensive Income (Loss) [Line Items]
Loss adjustment expenses	40.00%	40.00%	40.00%	40.00%
Percentage of other operating expenses	60.00%	60.00%	60.00%	60.00%

Percentage of amounts reclassified from AOCI for pension and post retirement benefits allocated to loss adjustment expenses.

No definition available.

Name:	thg_OtherComprehensiveIncomeLossPercentageOfAmountsReclassifiedFromAccumulatedOtherComprehensiveIncomeForPensionAndPostRetirementBenefitsAllocatedToLossAdjustmentExpenses
Namespace Prefix:	thg_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration