Fair Value (Schedule of Additional Information About Significant Unobservable Inputs Used in Fair Valuations of Level 3 Assets) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2019	Dec. 31, 2018
Fair Value [Line Items]
Fair Value	$ 6,559.4	$ 6,161.5
Equity securities, at fair value	504.7	464.4
Level 3 [Member]
Fair Value [Line Items]
Equity securities, at fair value	$ 1.1	$ 1.1
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Other investments	$ 3.5	$ 3.5
Alternative Investment, Valuation Technique [Extensible List]	us-gaap:ValuationTechniqueDiscountedCashFlowMember	us-gaap:ValuationTechniqueDiscountedCashFlowMember
Alternative Investment, Measurement Input [Extensible List]	us-gaap:MeasurementInputDiscountRateMember	us-gaap:MeasurementInputDiscountRateMember
Fair value measurement market multiples	1.0	1.0
Range	0.180	0.180
Level 3 [Member] \| Weighted Average [Member]
Fair Value [Line Items]
Fair value measurement market multiples	1.0	1.0
Range	0.180	0.180
Municipal [Member]
Fair Value [Line Items]
Fair Value	$ 865.5	$ 885.8
Municipal [Member] \| Discounted cash flow [Member] \| Level 3 [Member]
Fair Value [Line Items]
Fair Value	$ 20.5	$ 21.1
Municipal [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Measurement Input Discount For Credit Stress [Member]
Fair Value [Line Items]
Small issue size	1.30%	1.30%
Municipal [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Minimum [Member] \| Measurement Input, Discount for Small Issue Size [Member]
Fair Value [Line Items]
Small issue size	0.70%	0.70%
Municipal [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Minimum [Member] \| Measurement Input, Discount For Above-market coupon [Member]
Fair Value [Line Items]
Small issue size	0.30%	0.30%
Municipal [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Maximum [Member] \| Measurement Input, Discount for Small Issue Size [Member]
Fair Value [Line Items]
Small issue size	6.80%	6.80%
Municipal [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Maximum [Member] \| Measurement Input, Discount For Above-market coupon [Member]
Fair Value [Line Items]
Small issue size	0.50%	0.50%
Municipal [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Weighted Average [Member] \| Measurement Input, Discount for Small Issue Size [Member]
Fair Value [Line Items]
Small issue size	3.30%	3.40%
Municipal [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Weighted Average [Member] \| Measurement Input Discount For Credit Stress [Member]
Fair Value [Line Items]
Small issue size	1.30%	1.30%
Municipal [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Weighted Average [Member] \| Measurement Input, Discount For Above-market coupon [Member]
Fair Value [Line Items]
Small issue size	0.50%	0.50%
Corporate [Member] \| Discounted cash flow [Member] \| Level 3 [Member]
Fair Value [Line Items]
Fair Value	$ 0.7	$ 0.8
Corporate [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Measurement Input, Discount Rate [Member]
Fair Value [Line Items]
Small issue size	2.50%	2.50%
Above-market coupon	0.30%	0.30%
Corporate [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Weighted Average [Member] \| Measurement Input, Discount Rate [Member]
Fair Value [Line Items]
Small issue size	2.50%	2.50%
Above-market coupon	0.30%	0.30%
Commercial Mortgage Backed [Member] \| Discounted cash flow [Member] \| Level 3 [Member]
Fair Value [Line Items]
Fair Value	$ 12.8	$ 13.1
Commercial Mortgage Backed [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Measurement Input, Discount For Above-market coupon [Member]
Fair Value [Line Items]
Small issue size	0.50%	0.50%
Commercial Mortgage Backed [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Measurement Input, Discount For Lease Structure [Member]
Fair Value [Line Items]
Small issue size	0.30%	0.30%
Commercial Mortgage Backed [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Minimum [Member] \| Measurement Input, Discount for Small Issue Size [Member]
Fair Value [Line Items]
Small issue size	1.90%	1.90%
Commercial Mortgage Backed [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Maximum [Member] \| Measurement Input, Discount for Small Issue Size [Member]
Fair Value [Line Items]
Small issue size	3.10%	3.10%
Commercial Mortgage Backed [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Weighted Average [Member] \| Measurement Input, Discount for Small Issue Size [Member]
Fair Value [Line Items]
Small issue size	2.70%	2.70%
Commercial Mortgage Backed [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Weighted Average [Member] \| Measurement Input, Discount For Above-market coupon [Member]
Fair Value [Line Items]
Small issue size	0.50%	0.50%
Commercial Mortgage Backed [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Weighted Average [Member] \| Measurement Input, Discount For Lease Structure [Member]
Fair Value [Line Items]
Small issue size	0.30%	0.30%

Debt securities available for sale measurement input.

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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 debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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