SCHEDULE II CONDENSED FINANCIAL INFORMATION OF REGISTRANT PARENT COMPANY ONLY (BALANCE SHEETS) (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Assets
Fair Value	$ 7,985.3	$ 7,481.8
Equity securities - at fair value	130.9	241.9
Cash and cash equivalents	316.1	305.0
Other assets	504.0	493.0
Total assets	14,612.6	13,995.1
Liabilities
Debt	783.2	782.4
Total liabilities	12,147.0	11,661.4
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	1,939.2	1,913.1
Accumulated other comprehensive loss	(517.2)	(701.5)
Retained earnings	2,909.4	2,992.9
Treasury stock at cost (24.7 and 24.9 million shares)	(1,866.4)	(1,871.4)
Total shareholders’ equity	2,465.6	2,333.7	$ 3,136.0
Total liabilities and shareholders’ equity	14,612.6	13,995.1
The Hanover Insurance Group [Member]
Assets
Fair Value	283.5	290.3
Equity securities - at fair value	1.2	1.1
Cash and cash equivalents	38.3	43.2
Investments in subsidiaries	3,039.7	2,898.2
Current income tax receivable	2.8	2.1
Other assets	3.2	2.7
Total assets	3,394.0	3,263.8
Liabilities
Expenses and state taxes payable	9.9	12.4
Interest payable	10.3	10.3
Debt	908.2	907.4
Total liabilities	928.4	930.1
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	1,939.2	1,913.1
Accumulated other comprehensive loss	(517.2)	(701.5)
Retained earnings	2,909.4	2,992.9
Treasury stock at cost (24.7 and 24.9 million shares)	(1,866.4)	(1,871.4)
Total shareholders’ equity	2,465.6	2,333.7
Total liabilities and shareholders’ equity	3,394.0	3,263.8
The Hanover Insurance Group [Member] | Related Party [Member]
Assets
Net receivable from subsidiaries	$ 25.3	$ 26.2