Fair Value (Narrative) (Details) - USD ($)	12 Months Ended
Dec. 31, 2019	Dec. 31, 2020
Fair Value Measurements [Line Items]
Investments measured at net asset value based on an ownership interest in partners'	2.00%	2.00%
Transfer between Level 2 and Level 3	$ 0
Fair Value Measured Using NAV [Member] \| Limited Partnerships Interest [Member]
Fair Value Measurements [Line Items]
Investments measured at fair value using net asset value	183,600,000	$ 172,800,000
Level 3 [Member]
Fair Value Measurements [Line Items]
Investments measured at fair value using net asset value	3,500,000	4,000,000.0
Liabilities held	$ 0	$ 0

Fair Value Measurements [Line Items]

No definition available.

Name:	thg_FairValueMeasurementsLineItems
Namespace Prefix:	thg_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Fair value transfer between level 2 and level 3 amount.

+ References

No definition available.

+ Details

Name:	thg_FairValueTransferBetweenLevel2AndLevel3Amount
Namespace Prefix:	thg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Percentage Of Investments Not Included In Fair Value Of Financial Instruments

+ References

No definition available.

+ Details

Name:	thg_PercentageOfInvestmentsNotIncludedInFairValueOfFinancialInstruments
Namespace Prefix:	thg_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Fair value of investment other than investment in equity security, investment in debt security and equity method investment. Includes, but is not limited to, investment in certain entities that calculate net asset value per share. Example includes, but is not limited to, investment in hedge fund, venture capital fund, private equity fund, and real estate partnership or fund.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 6A
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=SL6742756-110258

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 35
-Paragraph 54B
-URI http://asc.fasb.org/extlink&oid=117332851&loc=SL7495116-110257

+ Details

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

- 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

- Details

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

- Details

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

- Details

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