SCHEDULE II CONDENSED FINANCIAL INFORMATION OF REGISTRANT PARENT COMPANY ONLY (BALANCE SHEETS) (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Assets
Fair Value	$ 7,481.8	$ 7,723.9
Equity securities - at fair value	241.9	661.3
Cash and cash equivalents	305.0	230.9
Other assets	493.0	606.3
Total assets	13,997.2	14,254.3
Liabilities
Debt	782.4	781.6
Total liabilities	11,671.6	11,109.4
Shareholders’ Equity
Preferred stock, par value $0.01 per share; 20.0 million shares authorized; none issued
Common stock, par value $0.01 per share; 300.0 million shares authorized; 60.5 million shares issued	0.6	0.6
Additional paid-in capital	1,913.1	1,887.2
Accumulated other comprehensive income (loss)	(705.5)	122.2
Retained earnings	2,988.8	2,983.2
Treasury stock at cost (24.9 and 25.0 million shares)	(1,871.4)	(1,848.3)
Total shareholders’ equity	2,325.6	3,144.9	$ 3,202.2
Total liabilities and shareholders’ equity	13,997.2	14,254.3
The Hanover Insurance Group [Member]
Assets
Fair Value	290.3	356.7
Equity securities - at fair value	1.1	1.2
Cash and cash equivalents	43.2	21.4
Investments in subsidiaries	2,890.1	3,665.2
Net receivable from subsidiaries	26.2	26.0
Current income tax receivable	2.1	6.7
Other assets	2.7	2.6
Total assets	3,255.7	4,079.8
Liabilities
Expenses and state taxes payable	12.4	18.0
Interest payable	10.3	10.3
Debt	907.4	906.6
Total liabilities	930.1	934.9
Shareholders’ Equity
Preferred stock, par value $0.01 per share; 20.0 million shares authorized; none issued
Common stock, par value $0.01 per share; 300.0 million shares authorized; 60.5 million shares issued	0.6	0.6
Additional paid-in capital	1,913.1	1,887.2
Accumulated other comprehensive income (loss)	(705.5)	122.2
Retained earnings	2,988.8	2,983.2
Treasury stock at cost (24.9 and 25.0 million shares)	(1,871.4)	(1,848.3)
Total shareholders’ equity	2,325.6	3,144.9
Total liabilities and shareholders’ equity	$ 3,255.7	$ 4,079.8