Condensed Financial Information of Registrant Parent Company Only Statements of Income (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Condensed Financial Statements, Captions [Line Items]
Income tax expense (benefit) from discontinued operations	$ 0.5	$ 2.5	$ 0.2
The Hanover Insurance Group, Inc
Condensed Financial Statements, Captions [Line Items]
Income tax expense (benefit) from discontinued operations	$ (0.1)	$ 2.8	$ 0.2

Tax effect allocated to a disposal group that is classified as a component of the entity reported as a separate component of income before extraordinary items. Includes the tax effects of the following: income (loss) from operations during the phase-out period, gain (loss) on disposal, provision (or any reversals of earlier provisions) for loss on disposal, and adjustments of a prior period gain (loss) on disposal.

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