Other Items - Additional Information (Details) $ in Millions	Jan. 02, 2019 USD ($)
Federal Home Loan Banks [Abstract]
Prepayment fees	$ 26
FHLB Interest rate	5.50%
Repayment of FHLB advance	$ 125
FHLB due date	2029

Prepayment fees paid on advances from FHLB.

No definition available.

Name:	thg_PrepaymentFeesPaidOnAdvancesFromFederalHomeLoanBank
Namespace Prefix:	thg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The year of maturity, when only one maturity date applies, for the advances from the Federal Home Loan Bank as of the Balance Sheet date.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=75038535&loc=d3e64711-112823

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6479118&loc=d3e64650-112822

+ Details

Name:	us-gaap_FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresDueDate
Namespace Prefix:	us-gaap_
Data Type:	us-types:gYearListItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of repayments and penalties incurred in the satisfaction or roll-over of Federal Home Loan Bank (FHLB) advance.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6479118&loc=d3e64650-112822

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=75038535&loc=d3e64711-112823

+ Details

Name:	us-gaap_FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresRepaymentAndPenalties
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Interest rate of advances made and reported by Federal Home Loan Bank (FHLBank).

+ References

No definition available.

+ Details

Name:	us-gaap_FederalHomeLoanBankAdvancesInterestRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- References

No definition available.

+ Details

Name:	us-gaap_FederalHomeLoanBanksAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration