Condensed Financial Information of Registrant Parent Company Only Balance Sheets (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Assets
Fixed maturities - at fair value (amortized cost of $131.1 and $189.3)	$ 6,952.2	$ 6,284.7
Equity securities - at fair value (cost of $1.0)	315.8	246.4
Cash and cash equivalents	564.8	820.4	290.4	316.5
Deferred income taxes	267.6	274.0
Other assets	511.8	515.5
Total assets	13,484.9	12,598.6
Liabilities
Debt	849.4	911.1
Total liabilities	10,889.5	10,114.6
Shareholders' Equity
Preferred stock, par value $0.01 per share, 20.0 million shares authorized, none issued
Common stock, par value $0.01 per share, 300.0 million shares authorized, 60.5 million shares issued	0.6	0.6
Additional paid-in capital	1,787.1	1,784.8
Accumulated other comprehensive income	325.8	210.4
Retained earnings	1,211.6	1,211.3
Treasury stock at cost (16.2 and 15.9 million)	(729.7)	(723.1)
Total shareholders' equity	2,595.4	2,484.0	2,437.4
Total liabilities and shareholders' equity	13,484.9	12,598.6
The Hanover Insurance Group, Inc
Assets
Fixed maturities - at fair value (amortized cost of $131.1 and $189.3)	139.3	196.9
Equity securities - at fair value (cost of $1.0)	1.0	1.0
Cash and cash equivalents	23.8	8.9	66.6	3.2
Investments in unconsolidated subsidiaries	2,763.8	2,616.5
Net receivable from subsidiaries	26.6	26.3
Deferred income taxes	20.7	19.2
Current income taxes	14.1	18.5
Loan receivable from subsidiary	300.0	300.0
Other assets	14.5	16.3
Total assets	3,303.8	3,203.6
Liabilities
Expenses and state taxes payable	20.7	31.4
Interest payable	9.6	9.6
Debt	678.1	678.6
Total liabilities	708.4	719.6
Shareholders' Equity
Preferred stock, par value $0.01 per share, 20.0 million shares authorized, none issued
Common stock, par value $0.01 per share, 300.0 million shares authorized, 60.5 million shares issued	0.6	0.6
Additional paid-in capital	1,787.1	1,784.8
Accumulated other comprehensive income	325.8	210.4
Retained earnings	1,211.6	1,211.3
Treasury stock at cost (16.2 and 15.9 million)	(729.7)	(723.1)
Total shareholders' equity	2,595.4	2,484.0
Total liabilities and shareholders' equity	$ 3,303.8	$ 3,203.6