Fair Value (Schedule of Additional Information About Significant Unobservable Inputs Used in Fair Valuations of Level 3) (Details) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Fair Value [Line Items]
Fair Value	$ 7,382.2	$ 7,723.9
Equity securities, at fair value	607.0	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]
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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	$ 1,135.9	$ 1,200.8
Municipal [Member] \| Discounted cash flow [Member] \| Level 3 [Member]
Fair Value [Line Items]
Fair Value	12.3	14.0
Corporate [Member]
Fair Value [Line Items]
Fair Value	3,911.5	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	796.4	824.4
Commercial Mortgage-Backed [Member] \| Discounted cash flow [Member] \| Level 3 [Member]
Fair Value [Line Items]
Fair Value	10.6	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
Fair value measurement market multiples		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
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
Revenue Market Multiples \| Equity Securities [Member] \| Market comparables [Member] \| Level 3 [Member]
Fair Value [Line Items]
Equity securities, at fair value	$ 3.6
Revenue Market Multiples \| Equity Securities [Member] \| Market comparables [Member] \| Level 3 [Member] \| Minimum [Member]
Fair Value [Line Items]
Fair value measurement market multiples	2.5
Revenue Market Multiples \| Equity Securities [Member] \| Market comparables [Member] \| Level 3 [Member] \| Maximum [Member]
Fair Value [Line Items]
Fair value measurement market multiples	8.3
Revenue Market Multiples \| Equity Securities [Member] \| Market comparables [Member] \| Level 3 [Member] \| Weighted Average [Member]
Fair Value [Line Items]
Fair value measurement market multiples	5.4
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	6.50%	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	1.90%	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	2.70%	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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Fair value of investment other than investment in equity security, investment in debt security and equity method investment. Includes, but is not limited to, investment in certain entities that calculate net asset value per share. Example includes, but is not limited to, investment in hedge fund, venture capital fund, private equity fund, and real estate partnership or fund.

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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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