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SCHEDULE II CONDENSED FINANCIAL INFORMATION OF REGISTRANT PARENT COMPANY ONLY (BALANCE SHEETS) (Parenthetical) (Details) - USD ($) $ / shares in Units, $ in Millions | Dec. 31, 2025 | Dec. 31, 2024
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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 | 25,100,000 | 24,600,000
The Hanover Insurance Group [Member] |  |
Condensed Financial Statements Captions [Line Items] |  |
Fixed maturities, amortized cost | $ 332.7 | $ 231.1
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 | 25,100,000 | 24,600,000

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