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, 2022	Dec. 31, 2021	Dec. 31, 2020
Condensed Financial Statements Captions [Line Items]
Net investment income	$ 296.3	$ 310.7	$ 265.1
Net realized gains (losses) from sales and other	(26.5)	4.6	17.9
Total revenues	5,468.6	5,227.8	4,824.8
Interest expense	34.1	34.0	37.1
Loss on repayment of debt			6.2
Other operating expenses	573.9	555.6	540.5
Total losses and expenses	5,324.6	4,706.5	4,380.0
Income tax benefit	(27.2)	(101.3)	(82.8)
Income from continuing operations	116.8	420.0	362.0
Net income	116.0	418.7	358.7
Other comprehensive income (loss), net of tax	(827.7)	(250.3)	219.9
Comprehensive income (loss)	(711.7)	168.4	578.6
Chaucer [Member]
Condensed Financial Statements Captions [Line Items]
Income (loss) from discontinued businesses, net of taxes		1.2	0.4
The Hanover Insurance Group [Member]
Condensed Financial Statements Captions [Line Items]
Net investment income	9.0	8.7	10.7
Net realized gains (losses) from sales and other	(1.2)	1.2	0.4
Other income	0.4
Total revenues	8.2	9.9	11.1
Interest expense	34.0	34.0	37.0
Employee benefit related expenses	2.7	1.5	2.3
Interest expense on loan from subsidiary	6.9	6.9	7.0
Loss on repayment of debt			6.2
Other operating expenses	6.8	5.8	9.8
Total losses and expenses	50.4	48.2	62.3
Net loss before income taxes and equity in income of subsidiaries	(42.2)	(38.3)	(51.2)
Income tax benefit	13.6	16.4	22.1
Equity in income of subsidiaries	144.6	440.6	387.2
Income from continuing operations	116.0	418.7	358.1
Income (loss) from discontinued businesses, net of taxes			0.6
Net income	116.0	418.7	358.7
Other comprehensive income (loss), net of tax	(827.7)	(250.3)	219.9
Comprehensive income (loss)	$ (711.7)	$ 168.4	$ 578.6