SCHEDULE II CONDENSED FINANCIAL INFORMATION OF REGISTRANT PARENT COMPANY ONLY (STATEMENTS OF INCOME) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018		Dec. 31, 2017	Dec. 31, 2016
Condensed Financial Statements Captions [Line Items]
Net investment income									$ 267.4		$ 243.9	$ 231.6
Net realized gains (losses) from sales and other									(2.7)		26.7	37.6
Total revenues	$ 1,102.8	$ 1,166.6	$ 1,133.4	$ 1,091.5	$ 1,092.5	$ 1,085.9	$ 1,053.3	$ 1,036.2	4,494.3		4,267.9	4,053.9
Interest expense									45.1		45.2	51.4
Loss from repayment of debt									1.9			88.3
Other operating expenses									522.1		509.5	498.6
Total losses and expenses									4,211.8		3,975.0	3,987.9
Income tax benefit									(43.5)		(76.8)	1.0
Income from continuing operations	2.1	104.0	82.4	50.5	71.9	59.5	56.3	28.4	239.0		216.1	67.0
Gain on sale of Chaucer business									131.9
Income from discontinued life business (net of income tax (expense) benefit									0.1		(16.8)	(1.0)
Net income	$ 123.6	$ 100.4	$ 99.3	$ 67.7	$ 51.5	$ 11.1	$ 78.4	$ 45.2	391.0		186.2	155.1
Income from discontinued operations, income tax (expense) benefit											(9.3)	(2.8)
Other comprehensive (loss) income, net of tax									(122.4)		44.8	8.9
Comprehensive income									268.6		231.0	164.0
Chaucer [Member]
Condensed Financial Statements Captions [Line Items]
Net investment income									54.9	[1]	52.0	45.7
Total revenues									912.4	[1]	911.7	891.4
Interest expense									3.8	[1]	3.3	3.5
Other operating expenses									115.0	[1]	109.2	113.7
Total losses and expenses									883.0	[1]	904.6	764.6
Income tax benefit									(4.7)	[1]	(21.7)	(37.2)
Income from discontinued life business (net of income tax (expense) benefit									20.0	[1]	(13.1)	89.1
The Hanover Insurance Group [Member]
Condensed Financial Statements Captions [Line Items]
Net investment income									11.8		7.6	4.9
Net realized gains (losses) from sales and other									(0.5)		(0.2)
Interest income from loan to subsidiary									0.3		0.2	22.4
Other income									0.5		0.5	0.5
Total revenues									12.1		8.1	27.8
Interest expense									37.8		38.1	46.1
Employee benefit related expenses									5.1		7.4	7.0
Interest expense on loan from subsidiary									6.8		7.0	6.3
Loss from repayment of debt									1.9			88.3
Other operating expenses									12.1		10.5	8.6
Total losses and expenses									63.7		63.0	156.3
Net loss before income taxes and equity in income of subsidiaries									(51.6)		(54.9)	(128.5)
Income tax benefit									21.4		44.4	75.6
Equity in income of subsidiaries									288.4		196.1	207.0
Income from continuing operations									258.2		185.6	154.1
Gain on sale of Chaucer business									131.9
Income from discontinued life business (net of income tax (expense) benefit									0.9		0.6	1.0
Net income									391.0		186.2	155.1
Income from discontinued operations, income tax (expense) benefit											(0.1)	1.7
Other comprehensive (loss) income, net of tax									(122.4)		44.8	8.9
Comprehensive income									268.6		$ 231.0	$ 164.0
The Hanover Insurance Group [Member] | Chaucer [Member]
Condensed Financial Statements Captions [Line Items]
Gain on sale of Chaucer business									$ 131.9

[1] 2018 reflects the results of operations for the period in which THG owned Chaucer, which concluded with the sale of the Chaucer business on December 28, 2018.