SCHEDULE II CONDENSED FINANCIAL INFORMATION OF REGISTRANT PARENT COMPANY ONLY (BALANCE SHEETS) (Parenthetical) (Details) - USD ($) $ / shares in Units, $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Condensed Financial Statements Captions [Line Items]
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, shares authorized	20,000,000.0	20,000,000.0
Preferred stock, issued	0	0
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	300,000,000.0	300,000,000.0
Common stock, shares issued	60,500,000	60,500,000
Treasury stock, shares	24,700,000	24,900,000
The Hanover Insurance Group [Member]
Condensed Financial Statements Captions [Line Items]
Fixed maturities, amortized cost	$ 296.1	$ 310.3
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, shares authorized	20,000,000.0	20,000,000.0
Preferred stock, issued	0	0
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	300,000,000.0	300,000,000.0
Common stock, shares issued	60,500,000	60,500,000
Treasury stock, shares	24,700,000	24,900,000