Fair Value (Schedule of Additional Information About Significant Unobservable Inputs Used in Fair Valuations of Level 3) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Fair Value [Line Items]
Fair Value	$ 6,161.5	$ 5,749.3
Equity securities, at fair value	464.4	576.2
Level 3 [Member]
Fair Value [Line Items]
Equity securities, at fair value	$ 1.1	$ 1.1
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Fair value measurement market multiples	1.0	1.0
Investments measured at fair value using net asset value	$ 3.5	$ 3.6
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Alternative Investment, Measurement Input [Extensible List]	us-gaap:MeasurementInputDiscountRateMember	us-gaap:MeasurementInputDiscountRateMember
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	$ 885.8	$ 934.0
Municipal [Member] \| Discounted cash flow [Member] \| Level 3 [Member]
Fair Value [Line Items]
Fair Value	$ 21.1	$ 24.6
Municipal [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Measurement Input Discount For Credit Stress
Fair Value [Line Items]
Small issue size	1.30%
Municipal [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Minimum [Member] \| Measurement Input Discount For Credit Stress
Fair Value [Line Items]
Small issue size		0.90%
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 Rate For Above Market Coupon
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 Credit Stress
Fair Value [Line Items]
Small issue size		1.50%
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 Rate For Above Market Coupon
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 Credit Stress
Fair Value [Line Items]
Small issue size	1.30%	1.20%
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	3.40%	3.50%
Municipal [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Weighted Average [Member] \| Measurement Input Discount Rate For Above Market Coupon
Fair Value [Line Items]
Small issue size	0.50%	0.40%
Corporate [Member] \| Discounted cash flow [Member] \| Level 3 [Member]
Fair Value [Line Items]
Fair Value	$ 0.8	$ 0.9
Corporate [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Measurement Input, Discount Rate
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
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	$ 13.1	$ 14.2
Commercial Mortgage Backed [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Measurement Input Discount Rate For Above Market Coupon
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 Rate For Lease Structure
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%	2.60%
Commercial Mortgage Backed [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Weighted Average [Member] \| Measurement Input Discount Rate For Above Market Coupon
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 Rate For Lease Structure
Fair Value [Line Items]
Small issue size	0.30%	0.30%

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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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