Condensed Financial Information of Registrant Parent Company Only - Additional Information (Detail) (The Hanover Insurance Group, Inc, USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
The Hanover Insurance Group, Inc
Supplementary Insurance Information, by Segment [Line Items]
Payment for investment assets	$ 17.9	$ 97.8	$ 69.4
Dividends received from unconsolidated subsidiaries	$ 1.0	$ 1.6