Condensed Financial Information of Registrant Parent Company Only Statements of Income (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Condensed Financial Statements, Captions [Line Items]
Net investment income									$ 276.6	$ 258.2	$ 247.2
Net realized gains from sales and other									31.4	35.0	43.6
Net other-than-temporary impairment losses on investments recognized in earnings									(7.8)	(6.9)	(13.9)
Total net realized investment gains									23.6	28.1	29.7
Total revenues	1,184.0	1,157.7	1,127.2	1,121.8	1,135.9	1,108.0	853.9	833.8	4,590.7	3,931.6	3,152.2
Interest expense									61.9	55.0	44.3
Costs related to acquired businesses									5.1	2.3	2.0
Other operating expenses									587.6	525.3	445.2
Total expenses									4,562.0	3,910.0	2,946.6
Income tax benefit									(2.3)	0.6	(5.7)
Income from continuing operations	(55.3)	40.9	9.8	50.7	46.4	(10.0)	(32.8)	27.9	46.1	31.5	149.6
Net income	(55.0)	40.4	20.8	49.7	49.6	(10.0)	(32.2)	29.3	55.9	36.7	151.2
The Hanover Insurance Group, Inc
Condensed Financial Statements, Captions [Line Items]
Net investment income									6.6	10.5	12.2
Net realized gains from sales and other									2.5	8.7	0.6
Net other-than-temporary impairment losses on investments recognized in earnings											(0.3)
Total net realized investment gains									2.5	8.7	0.3
Interest income from loan to subsidiary									22.5	10.5
Other Income									0.1
Total revenues									31.7	29.7	12.5
Interest expense									48.6	41.3	35.7
Employee benefit related expenses									5.3	6.9	6.9
Costs related to acquired businesses									2.6	16.4
Loss on derivative instruments										11.3
Other operating expenses									8.0	8.1	5.9
Total expenses									64.5	84.0	48.5
Net income (loss) before income taxes and equity in income of unconsolidated subsidiaries									(32.8)	(54.3)	(36.0)
Income tax benefit									40.5	31.1	25.1
Equity in income of unconsolidated subsidiaries									47.5	54.7	160.2
Income from continuing operations									55.2	31.5	149.3
Income from discontinued operations (net of income tax expense (benefit) of $(0.1), $2.8 and $0.2 in 2012, 2011 and 2010)									0.7	5.2	1.9
Net income									$ 55.9	$ 36.7	$ 151.2