Accelerated share repurchases final settlement of shares received.

No definition available.

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Namespace Prefix:	thg_
Data Type:	xbrli:sharesItemType
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- Definition

Accelerated share repurchases initial delivery of shares received.

+ References

No definition available.

+ Details

Name:	thg_AcceleratedShareRepurchasesInitialDeliveryOfSharesReceived
Namespace Prefix:	thg_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Share repurchase program percentage of shares repurchased.

+ References

No definition available.

+ Details

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Namespace Prefix:	thg_
Data Type:	num:percentItemType
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- Definition

Amount of cash receipt from (payment to) bank; or stock received from (issuance to) bank in the settlement of the accelerated share repurchase agreement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 30
-Section 25
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6405686&loc=d3e22802-112653

+ Details

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

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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- Details

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Namespace Prefix:
Data Type:	na
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- Details

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Quarterly Results of Operations (Narrative) (Details) - October 2020 Accelerated Share Repurchase Agreement [Member] - USD ($) $ in Millions	Jan. 29, 2021	Oct. 29, 2020	Dec. 31, 2020
Class Of Stock [Line Items]
Amount paid by company			$ 100.0
Initial delivery of shares of common stock received		800,000
Shares repurchased under repurchase agreement, percentage		80.00%
Subsequent Event [Member]
Class Of Stock [Line Items]
Common stock shares received as final settlement of shares repurchased	45,000