Fair Value (Fair Values of Financial Instruments) (Details) - USD ($) $ in Millions	Sep. 30, 2025	Dec. 31, 2024
Carrying Value [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 915.7	$ 435.5
Total financial instruments	10,981.4	9,566.0
Carrying Value [Member] \| Income Approach Valuation Technique [Member] \| Fair Value Through AOCI [Member] \| Debt Securities [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Fair value of investments	9,498.6	8,542.2
Carrying Value [Member] \| Income Approach Valuation Technique [Member] \| Fair Value Through Net Income [Member] \| Other Investments [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Fair value of investments	90.6	86.9
Carrying Value [Member] \| Income Approach Valuation Technique [Member] \| Fair Value Through Net Income [Member] \| Equity Securities [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Fair value of investments	183.2	157.7
Carrying Value [Member] \| Cost Approach Valuation Technique [Member] \| Amortized Cost/Cost [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	915.7	435.5
Debt	1,279.8	784.1
Carrying Value [Member] \| Cost Approach Valuation Technique [Member] \| Amortized Cost/Cost [Member] \| Other Investments [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Fair value of investments	293.3	343.7
Estimated Fair Value [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	915.7	435.5
Total financial instruments	10,971.7	9,546.5
Debt	1,264.5	744.4
Estimated Fair Value [Member] \| Income Approach Valuation Technique [Member] \| Fair Value Through AOCI [Member] \| Debt Securities [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Fair value of investments	9,498.6	8,542.2
Estimated Fair Value [Member] \| Income Approach Valuation Technique [Member] \| Fair Value Through Net Income [Member] \| Other Investments [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Fair value of investments	90.6	86.9
Estimated Fair Value [Member] \| Income Approach Valuation Technique [Member] \| Fair Value Through Net Income [Member] \| Equity Securities [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Fair value of investments	183.2	157.7
Estimated Fair Value [Member] \| Cost Approach Valuation Technique [Member] \| Amortized Cost/Cost [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	915.7	435.5
Debt	1,264.5	744.4
Estimated Fair Value [Member] \| Cost Approach Valuation Technique [Member] \| Amortized Cost/Cost [Member] \| Other Investments [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Fair value of investments	$ 283.6	$ 324.2

HANOVER INSURANCE GROUP, INC. 10-Q