Fair Value (Narrative) (Detail) (USD $)	6 Months Ended
Jun. 30, 2014	Jun. 30, 2013
Fair Value Disclosures [Abstract]
Transfer between Level 1 and Level 2	$ 0	$ 0

Fair Value Transfers Between Level 1 And Level 2 Amount

No definition available.

Name:	thg_FairValueTransfersBetweenLevel1AndLevel2Amount
Namespace Prefix:	thg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration