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, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Condensed Financial Statements Captions [Line Items]
Net investment income									$ 265.1	$ 281.3	$ 267.4
Net realized gains (losses) from sales and other									17.9	4.9	(2.7)
Total revenues	$ 1,297.0	$ 1,249.8	$ 1,224.3	$ 1,056.2	$ 1,257.9	$ 1,214.7	$ 1,198.6	$ 1,219.5	4,827.3	4,890.7	4,494.3
Interest expense									37.1	37.5	45.1
Loss on repayment of debt									6.2		28.2
Other operating expenses									543.0	538.9	522.1
Total losses and expenses									4,382.5	4,368.6	4,211.8
Income tax benefit									(82.8)	(93.1)	(43.5)
Income from continuing operations	165.9	119.5	115.3	(38.7)	110.2	111.2	85.0	122.6	362.0	429.0	239.0
Gain on sale of Chaucer business										(1.2)	131.9
Net income	$ 164.6	$ 118.9	$ 115.2	$ (40.0)	$ 109.8	$ 118.9	$ 74.0	$ 122.4	358.7	425.1	391.0
Other comprehensive income (loss), net of tax									219.9	267.6	(122.4)
Comprehensive income									578.6	692.7	268.6
Chaucer [Member]
Condensed Financial Statements Captions [Line Items]
Net investment income											54.9
Total revenues										6.7	912.4
Interest expense											3.8
Other operating expenses											115.0
Total losses and expenses											883.0
Income (loss) from discontinued businesses, net of taxes									0.4	1.6	20.0
The Hanover Insurance Group [Member]
Condensed Financial Statements Captions [Line Items]
Net investment income									10.7	19.8	11.8
Net realized gains (losses) from sales and other									0.4	5.3	(0.5)
Other income										0.4	0.8
Total revenues									11.1	25.5	12.1
Interest expense									37.0	37.4	37.8
Employee benefit related expenses									2.3	5.0	5.1
Interest expense on loan from subsidiary									7.0	6.9	6.8
Loss on repayment of debt									6.2		1.9
Other operating expenses									9.8	7.1	12.1
Total losses and expenses									62.3	56.4	63.7
Net loss before income taxes and equity in income of subsidiaries									(51.2)	(30.9)	(51.6)
Income tax benefit									22.1	25.6	21.4
Equity in income of subsidiaries									387.2	432.4	288.4
Income from continuing operations									358.1	427.1	258.2
Gain on sale of Chaucer business										(2.1)	131.9
Income (loss) from discontinued businesses, net of taxes									0.6	0.1	0.9
Net income									358.7	425.1	391.0
Other comprehensive income (loss), net of tax									219.9	267.6	(122.4)
Comprehensive income									$ 578.6	692.7	268.6
The Hanover Insurance Group [Member] | Chaucer [Member]
Condensed Financial Statements Captions [Line Items]
Gain on sale of Chaucer business										$ (2.1)	$ 131.9