SCHEDULE II CONDENSED FINANCIAL INFORMATION OF REGISTRANT PARENT COMPANY ONLY (STATEMENTS OF INCOME AND COMPREHENSIVE INCOME) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Condensed Financial Statements Captions [Line Items]
Net investment income	$ 332.1	$ 296.3	$ 310.7
Net realized gains (losses) from sales and other	(8.9)	(26.5)	4.6
Total revenues	5,993.5	5,468.6	5,227.8
Interest expense	34.1	34.1	34.0
Other operating expenses	607.7	573.9	555.6
Total losses and expenses	5,952.4	5,324.6	4,706.5
Income tax benefit	(7.6)	(27.2)	(101.3)
Income from continuing operations	33.5	116.8	420.0
Net income	35.3	116.0	422.8
Other comprehensive income (loss), net of tax	184.3	(810.7)	(246.1)
Comprehensive income (loss)	219.6	(694.7)	176.7
The Hanover Insurance Group [Member]
Condensed Financial Statements Captions [Line Items]
Net investment income	11.5	9.0	8.7
Net realized gains (losses) from sales and other	(1.1)	(1.2)	1.2
Other income	0.5	0.4
Total revenues	10.9	8.2	9.9
Interest expense	34.1	34.0	34.0
Employee benefit related expenses	3.9	2.7	1.5
Other operating expenses	4.8	6.8	5.8
Total losses and expenses	49.8	50.4	48.2
Net loss before income taxes and equity in income of subsidiaries	(38.9)	(42.2)	(38.3)
Income tax benefit	10.8	13.6	16.4
Equity in income of subsidiaries	62.2	144.6	444.7
Income from continuing operations	34.1	116.0	422.8
Income (loss) from discontinued businesses, net of taxes	1.2
Net income	35.3	116.0	422.8
Other comprehensive income (loss), net of tax	184.3	(810.7)	(246.1)
Comprehensive income (loss)	219.6	(694.7)	176.7
The Hanover Insurance Group [Member] | Related Party [Member]
Condensed Financial Statements Captions [Line Items]
Interest expense on loan from subsidiary	$ 7.0	$ 6.9	$ 6.9