Fair Value (Schedule of Additional Information About Significant Unobservable Inputs Used in Fair Valuations of Level 3) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Fair Value [Line Items]
Fair Value	$ 7,454.4	$ 6,687.1
Equity securities, at fair value	598.5	575.7
Level 3 [Member]
Fair Value [Line Items]
Fair Value	$ 4.0	$ 3.5
Alternative Investment, Valuation Technique [Extensible List]	us-gaap:ValuationTechniqueDiscountedCashFlowMember	us-gaap:ValuationTechniqueDiscountedCashFlowMember
Alternative Investment, Measurement Input [Extensible List]	us-gaap:MeasurementInputDiscountRateMember	us-gaap:MeasurementInputDiscountRateMember
Range	0.162	0.180
Level 3 [Member] \| Weighted Average [Member]
Fair Value [Line Items]
Range	0.162	0.180
Municipal [Member]
Fair Value [Line Items]
Fair Value	$ 1,103.7	$ 833.5
Municipal [Member] \| Income Approach Valuation Technique [Member] \| Level 3 [Member]
Fair Value [Line Items]
Fair Value	$ 7.0	$ 12.1
Municipal [Member] \| Income Approach Valuation Technique [Member] \| Level 3 [Member] \| Measurement Input Discount For Credit Stress
Fair Value [Line Items]
Small issue size	0.20%
Municipal [Member] \| Income Approach Valuation Technique [Member] \| Level 3 [Member] \| Measurement Input Discount Rate For Above Market Coupon
Fair Value [Line Items]
Small issue size		0.50%
Municipal [Member] \| Income Approach Valuation Technique [Member] \| Level 3 [Member] \| Minimum [Member] \| Measurement Input, Discount for Small Issue Size [Member]
Fair Value [Line Items]
Small issue size	0.70%	0.70%
Municipal [Member] \| Income Approach Valuation Technique [Member] \| Level 3 [Member] \| Maximum [Member] \| Measurement Input, Discount for Small Issue Size [Member]
Fair Value [Line Items]
Small issue size	6.80%	6.80%
Municipal [Member] \| Income Approach Valuation Technique [Member] \| Level 3 [Member] \| Weighted Average [Member] \| Measurement Input Discount For Credit Stress
Fair Value [Line Items]
Small issue size	0.20%
Municipal [Member] \| Income Approach Valuation Technique [Member] \| Level 3 [Member] \| Weighted Average [Member] \| Measurement Input, Discount for Small Issue Size [Member]
Fair Value [Line Items]
Small issue size	5.00%	4.30%
Municipal [Member] \| Income Approach Valuation Technique [Member] \| Level 3 [Member] \| Weighted Average [Member] \| Measurement Input Discount Rate For Above Market Coupon
Fair Value [Line Items]
Small issue size		0.50%
Corporate [Member]
Fair Value [Line Items]
Fair Value	$ 4,111.3	$ 3,811.2
Corporate [Member] \| Income Approach Valuation Technique [Member] \| Level 3 [Member]
Fair Value [Line Items]
Fair Value	$ 0.5	$ 0.6
Corporate [Member] \| Income Approach Valuation Technique [Member] \| Level 3 [Member] \| Measurement Input, Discount Rate
Fair Value [Line Items]
Small issue size	2.50%	2.50%
Above-market coupon	0.30%	0.30%
Corporate [Member] \| Income Approach Valuation Technique [Member] \| Level 3 [Member] \| Weighted Average [Member] \| Measurement Input, Discount Rate
Fair Value [Line Items]
Small issue size	2.50%	2.50%
Above-market coupon	0.30%	0.30%
Commercial Mortgage-Backed [Member]
Fair Value [Line Items]
Fair Value	$ 760.0	$ 691.9
Commercial Mortgage-Backed [Member] \| Income Approach Valuation Technique [Member] \| Level 3 [Member]
Fair Value [Line Items]
Fair Value	$ 12.4	$ 12.7
Commercial Mortgage-Backed [Member] \| Income Approach Valuation Technique [Member] \| Level 3 [Member] \| Measurement Input Discount For Credit Stress
Fair Value [Line Items]
Small issue size	0.20%
Commercial Mortgage-Backed [Member] \| Income Approach Valuation Technique [Member] \| Level 3 [Member] \| Measurement Input Discount Rate For Above Market Coupon
Fair Value [Line Items]
Small issue size	0.50%	0.50%
Commercial Mortgage-Backed [Member] \| Income Approach Valuation Technique [Member] \| Level 3 [Member] \| Measurement Input Discount Rate For Lease Structure
Fair Value [Line Items]
Small issue size	0.30%	0.30%
Commercial Mortgage-Backed [Member] \| Income Approach Valuation Technique [Member] \| Level 3 [Member] \| Minimum [Member] \| Measurement Input, Discount for Small Issue Size [Member]
Fair Value [Line Items]
Small issue size	1.90%	1.90%
Commercial Mortgage-Backed [Member] \| Income Approach Valuation Technique [Member] \| Level 3 [Member] \| Maximum [Member] \| Measurement Input, Discount for Small Issue Size [Member]
Fair Value [Line Items]
Small issue size	3.10%	3.10%
Commercial Mortgage-Backed [Member] \| Income Approach Valuation Technique [Member] \| Level 3 [Member] \| Weighted Average [Member] \| Measurement Input Discount For Credit Stress
Fair Value [Line Items]
Small issue size	0.20%
Commercial Mortgage-Backed [Member] \| Income Approach Valuation Technique [Member] \| Level 3 [Member] \| Weighted Average [Member] \| Measurement Input, Discount for Small Issue Size [Member]
Fair Value [Line Items]
Small issue size	2.70%	2.70%
Commercial Mortgage-Backed [Member] \| Income Approach Valuation Technique [Member] \| Level 3 [Member] \| Weighted Average [Member] \| Measurement Input Discount Rate For Above Market Coupon
Fair Value [Line Items]
Small issue size	0.50%	0.50%
Commercial Mortgage-Backed [Member] \| Income Approach Valuation Technique [Member] \| Level 3 [Member] \| Weighted Average [Member] \| Measurement Input Discount Rate For Lease Structure
Fair Value [Line Items]
Small issue size	0.30%	0.30%
Equity Securities [Member] \| Market Comparables [Member] \| Level 3 [Member]
Fair Value [Line Items]
Equity securities, at fair value	$ 1.1	$ 1.1
Fair value measurement market multiples	1.0	1.0
Equity Securities [Member] \| Market Comparables [Member] \| Level 3 [Member] \| Weighted Average [Member]
Fair Value [Line Items]
Fair value measurement market multiples	1.0	1.0
Equity Securities [Member] \| Internal Price [Member] \| Level 3 [Member]
Fair Value [Line Items]
Equity securities, at fair value	$ 4.4	$ 1.0
Fair value measurement market multiples	5.60	1.27
Equity Securities [Member] \| Internal Price [Member] \| Level 3 [Member] \| Weighted Average [Member]
Fair Value [Line Items]
Fair value measurement market multiples	5.60	1.27

Debt securities available for sale measurement input.

No definition available.

Name:	thg_DebtSecuritiesAvailableForSaleMeasurementInput1
Namespace Prefix:	thg_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Fair value inputs discount rate for above market coupon.

+ References

No definition available.

+ Details

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

- Definition

Fair value inputs discount rate for small issue size.

+ References

No definition available.

+ Details

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

- Definition

Fair Value [Line Items]

+ References

No definition available.

+ Details

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

- 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

Value of input used to measure alternative investment.

+ References

+ Details

Name:	us-gaap_AlternativeInvestmentMeasurementInput
Namespace Prefix:	us-gaap_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

- Definition

Indicates measurement input for alternative investment.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)(1)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 31
-URI http://asc.fasb.org/extlink&oid=75031198&loc=d3e14210-108612

+ Details

Name:	us-gaap_AlternativeInvestmentMeasurementInputExtensibleList
Namespace Prefix:	us-gaap_
Data Type:	srt-types:extensibleListItemType
Balance Type:	na
Period Type:	instant

- Definition

Indicates valuation technique for measuring alternative investment.

+ References

+ Details

Name:	us-gaap_AlternativeInvestmentValuationTechniqueExtensibleList
Namespace Prefix:	us-gaap_
Data Type:	srt-types:extensibleListItemType
Balance Type:	na
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 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(6))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121558606&loc=SL82898722-210454

Reference 3: 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=121553693&loc=d3e26610-111562

Reference 4: 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=121645371&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 investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=99377092&loc=SL75136599-209740

+ Details

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

- Definition

Value of input used to measure investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

+ References

+ Details

Name:	us-gaap_EquitySecuritiesFvNiMeasurementInput
Namespace Prefix:	us-gaap_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

- Details

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

- Details

Name:	srt_RangeAxis=srt_WeightedAverageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_ValuationTechniqueAxis=us-gaap_ValuationTechniqueDiscountedCashFlowMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MeasurementInputTypeAxis=thg_MeasurementInputDiscountForCreditStressMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MeasurementInputTypeAxis=thg_MeasurementInputDiscountRateForAboveMarketCouponMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MeasurementInputTypeAxis=thg_MeasurementInputDiscountForSmallIssueSizeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_MeasurementInputTypeAxis=us-gaap_MeasurementInputDiscountRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_MeasurementInputTypeAxis=thg_MeasurementInputDiscountRateForLeaseStructureMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_ValuationTechniqueAxis=us-gaap_MarketApproachValuationTechniqueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ValuationTechniqueAxis=thg_InternalModelPricingApproachValuationTechniqueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: