Accelerated share repurchases initial delivery of shares received.

No definition available.

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- Definition

Share repurchase program percentage of shares repurchased.

+ References

No definition available.

+ Details

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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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- Definition

The cash inflow associated with the amount received from the sale of a portion of the company's business, for example a segment, division, branch or other business, during the period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 12
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3179-108585

+ Details

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- Definition

Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event.

+ References

No definition available.

+ Details

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Data Type:	na
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- Details

Name:	us-gaap_ShareRepurchaseProgramAxis=thg_AcceleratedShareRepurchaseProgramMember
Namespace Prefix:
Data Type:	na
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Subsequent Events (Narrative) (Details) - Subsequent Event [Member] - USD ($) shares in Millions, $ in Millions	Feb. 14, 2019	Jan. 02, 2019
Subsequent Event [Line Items]
Amount paid by company		$ 250.0
Initial delivery of shares of common stock received		1.8
Chaucer Irish Entity[Member]
Subsequent Event [Line Items]
Proceeds from sale of businesses segment	$ 28.0
Accelerated Share Repurchase Agreement [Member]
Subsequent Event [Line Items]
Shares repurchased under repurchase agreement, percentage		80.00%