Condensed Financial Information of Registrant Parent Company Only Statements Of Cash Flows (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Cash Flows From Operating Activities
Net income	$ 55.9	$ 36.7	$ 151.2
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Net realized investment gains	(22.8)	(16.2)	(29.3)
Deferred income tax expense (benefit)	(35.5)	(9.2)	50.1
Change in expenses and taxes payable	87.6	48.2	(85.0)
Net cash provided by (used in) operating activities	408.2	221.7	83.6
Cash Flows From Investing Activities
Proceeds from disposals and maturities of fixed maturities	1,684.8	1,624.2	1,376.2
Purchase of fixed maturities	(2,170.4)	(1,688.0)	(1,401.5)
Net cash used for business acquisitions		287.7	(13.3)
Net cash provided by (used in) investing activities	(562.8)	182.8	(98.8)
Cash flow from financing activities
Proceeds from debt borrowings	7.4	325.4	207.5
Repurchases of debt	(73.1)	(86.8)	(38.5)
Dividends paid to shareholders	(55.1)	(50.9)	(47.2)
Repurchase of common stock	(20.0)	(21.7)	(134.7)
Proceeds from exercise of employee stock options	2.6	3.9	12.0
Other financing activities	(0.4)	(0.5)
Net cash provided by (used in) financing activities	(133.4)	127.3	(8.6)
Net change in cash and cash equivalents	(255.9)	527.9	(23.8)
Cash and cash equivalents, beginning of year	820.4	290.4	316.5
Cash and cash equivalents, end of year	564.8	820.4	290.4
The Hanover Insurance Group, Inc
Cash Flows From Operating Activities
Net income	55.9	36.7	151.2
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Gain on disposal of discontinued operations	(0.7)	(5.2)	(1.8)
Equity in net income of unconsolidated subsidiaries	(24.9)	(54.7)	(160.2)
Net realized investment gains	(2.5)	(8.7)	(0.3)
Loss on derivative instruments		11.3
Deferred income tax expense (benefit)	3.9	(3.4)	104.3
Change in expenses and taxes payable	6.2	(11.7)	(8.2)
Change in net payable from subsidiaries	9.3	(0.5)	8.7
Other, net	2.7	6.5	0.7
Net cash provided by (used in) operating activities	49.9	(29.7)	94.4
Cash Flows From Investing Activities
Proceeds from disposals and maturities of fixed maturities	44.2	436.3	177.3
Purchase of fixed maturities	(2.2)	(148.7)	(167.4)
Net cash used for business acquisitions	(3.4)	(468.4)	(29.5)
Other investing activities		(1.9)	(1.0)
Net cash provided by (used in) investing activities	38.6	(182.7)	(20.6)
Cash flow from financing activities
Proceeds from debt borrowings		296.0	198.0
Repurchases of debt	(0.7)	(72.1)	(38.5)
Dividends paid to shareholders	(55.1)	(50.9)	(47.2)
Repurchase of common stock	(20.0)	(21.7)	(134.7)
Proceeds from exercise of employee stock options	2.6	3.9	12.0
Other financing activities	(0.4)	(0.5)
Net cash provided by (used in) financing activities	(73.6)	154.7	(10.4)
Net change in cash and cash equivalents	14.9	(57.7)	63.4
Cash and cash equivalents, beginning of year	8.9	66.6	3.2
Cash and cash equivalents, end of year	$ 23.8	$ 8.9	$ 66.6