Fair Value (Schedule of Additional Information About Significant Unobservable Inputs Used in Fair Valuations of Level 3) (Details) $ in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2022 USD ($)	Dec. 31, 2021 USD ($)
Fair Value [Line Items]
Fair Value	$ 7,200.9	$ 7,723.9
Equity securities, at fair value	399.5	661.3
Level 3 [Member]
Fair Value [Line Items]
Fair Value	$ 4.3	$ 4.3
Alternative Investment, Valuation Technique [Extensible Enumeration]	Discounted cash flow [Member]	Discounted cash flow [Member]
Alternative Investment, Measurement Input [Extensible Enumeration]	us-gaap:MeasurementInputDiscountRateMember	us-gaap:MeasurementInputDiscountRateMember
Discounted cash flow [Member] \| Level 3 [Member]
Fair Value [Line Items]
Range	0.161	0.161
Discounted cash flow [Member] \| Level 3 [Member] \| Weighted Average [Member]
Fair Value [Line Items]
Range	0.161	0.161
Municipal [Member]
Fair Value [Line Items]
Fair Value	$ 1,051.8	$ 1,200.8
Municipal [Member] \| Discounted cash flow [Member] \| Level 3 [Member]
Fair Value [Line Items]
Fair Value	10.0	14.0
Corporate [Member]
Fair Value [Line Items]
Fair Value	3,616.8	4,090.1
Corporate [Member] \| Discounted cash flow [Member] \| Level 3 [Member]
Fair Value [Line Items]
Fair Value	0.1	0.1
Commercial Mortgage-Backed [Member]
Fair Value [Line Items]
Fair Value	825.8	824.4
Commercial Mortgage-Backed [Member] \| Discounted cash flow [Member] \| Level 3 [Member]
Fair Value [Line Items]
Fair Value	7.6	11.3
Equity Securities [Member] \| Internal Model Pricing Approach Valuation Technique \| Level 3 [Member]
Fair Value [Line Items]
Equity securities, at fair value	$ 8.8	$ 8.8
Fair value measurement market multiples	11.18	11.18
Equity Securities [Member] \| Internal Model Pricing Approach Valuation Technique \| Level 3 [Member] \| Weighted Average [Member]
Fair Value [Line Items]
Fair value measurement market multiples	11.18	11.18
Net Tangible Asset [Member] \| Equity Securities [Member] \| Market comparables [Member] \| Level 3 [Member]
Fair Value [Line Items]
Equity securities, at fair value	$ 1.2	$ 1.3
Measurement Input, Discount for Small Issue Size [Member] \| Municipal [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Minimum [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	4.50%	4.50%
Measurement Input, Discount for Small Issue Size [Member] \| Municipal [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Maximum [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	6.80%	6.80%
Measurement Input, Discount for Small Issue Size [Member] \| Municipal [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Weighted Average [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	6.60%	6.50%
Measurement Input, Discount for Small Issue Size [Member] \| Corporate [Member] \| Discounted cash flow [Member] \| Level 3 [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	2.50%	2.50%
Measurement Input, Discount for Small Issue Size [Member] \| Corporate [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Weighted Average [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	2.50%	2.50%
Measurement Input, Discount for Small Issue Size [Member] \| Commercial Mortgage-Backed [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Minimum [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	2.30%	1.90%
Measurement Input, Discount for Small Issue Size [Member] \| Commercial Mortgage-Backed [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Maximum [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	3.10%	3.10%
Measurement Input, Discount for Small Issue Size [Member] \| Commercial Mortgage-Backed [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Weighted Average [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	2.90%	2.70%
Measurement Input, Discount For Above-market coupon [Member] \| Corporate [Member] \| Discounted cash flow [Member] \| Level 3 [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	0.30%	0.30%
Measurement Input, Discount For Above-market coupon [Member] \| Corporate [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Weighted Average [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	0.30%	0.30%
Measurement Input, Discount For Above-market coupon [Member] \| Commercial Mortgage-Backed [Member] \| Discounted cash flow [Member] \| Level 3 [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	0.50%	0.50%
Measurement Input, Discount For Above-market coupon [Member] \| Commercial Mortgage-Backed [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Weighted Average [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	0.50%	0.50%
Measurement Input, Discount For Lease Structure [Member] \| Commercial Mortgage-Backed [Member] \| Discounted cash flow [Member] \| Level 3 [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	0.30%	0.30%
Measurement Input, Discount For Lease Structure [Member] \| Commercial Mortgage-Backed [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Weighted Average [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	0.30%	0.30%

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Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI), classified as current.

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