Condensed Financial Information of Registrant Parent Company Only Balance Sheets (Parenthetical) (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Condensed Financial Statements, Captions [Line Items]
Fixed maturities, amortized cost	$ 6,529.5	$ 6,008.7
Equity securities, cost	299.0	239.9
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, shares authorized	20.0	20.0
Preferred stock, shares issued
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	300.0	300.0
Common stock, shares issued	60.5	60.5
Treasury stock, shares	16.2	15.9
The Hanover Insurance Group, Inc
Condensed Financial Statements, Captions [Line Items]
Fixed maturities, amortized cost	131.1	189.3
Equity securities, cost	$ 1.0	$ 1.0
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, shares authorized	20.0	20.0
Preferred stock, shares issued
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	300.0	300.0
Common stock, shares issued	60.5	60.5
Treasury stock, shares	16.2