Investments (Rollforwards Of Cumulative Amounts Related To Credit Loss Portion Of OTTI Losses) (Details) (USD $) In Millions	3 Months Ended	9 Months Ended
Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Investments [Abstract]
Credit losses, beginning of period	$ 14.8	$ 19.5	$ 16.7	$ 20.0
Credit losses for which an OTTI was not previously recognized				0.3
Additional credit losses on securities for which an OTTI was previously recognized	0.4	0.2	0.6	2.4
Reductions for securities sold or matured during the period	(0.3)	(2.9)	(1.6)	(5.9)
Reduction for securities reclassified as intend to sell		(0.4)	(0.8)	(0.4)
Credit losses, end of period	$ 14.9	$ 16.4	$ 14.9	$ 16.4

OTTI Credit Losses Recognized In Earnings Additions Additional Credit Losses

Name:	thg_OttiCreditLossesRecognizedInEarningsAdditionsAdditionalCreditLosses
Namespace Prefix:	thg
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

OTTI Credit Losses Recognized In Earnings Additions No Previous Impairment

Name:	thg_OttiCreditLossesRecognizedInEarningsAdditionsNoPreviousImpairment
Namespace Prefix:	thg
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

OTTI Credit Losses Recognized In Earnings On Debt Securities

Name:	thg_OttiCreditLossesRecognizedInEarningsOnDebtSecurities
Namespace Prefix:	thg
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

OTTI Credit Losses Recognized In Earnings Reductions Securities Sold

Name:	thg_OttiCreditLossesRecognizedInEarningsReductionsSecuritiesSold
Namespace Prefix:	thg
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Reduction for Securities Reclassified As Intend To Sell As Intend To Sell

Name:	thg_ReductionForSecuritiesReclassifiedAsIntendToSellAsIntendToSell
Namespace Prefix:	thg
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Name:	us-gaap_InvestmentsAbstract
Namespace Prefix:	us-gaap
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration