Fair Value (Schedule of Additional Information About Significant Unobservable Inputs Used in Fair Valuations of Level 3) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Fair Value [Line Items]
Fair Value	$ 44.3	$ 46.6	$ 54.9	$ 55.8
Municipal [Member]
Fair Value [Line Items]
Fair Value	25.3	26.8	30.8	31.0
Municipal [Member] | Discounted cash flow [Member] | Level 3 [Member]
Fair Value [Line Items]
Fair Value	$ 25.3	$ 26.8
Municipal [Member] | Discounted cash flow [Member] | Level 3 [Member] | Discount For Credit Stress [Member]
Fair Value [Line Items]
Range	1.30%
Municipal [Member] | Discounted cash flow [Member] | Level 3 [Member] | Minimum [Member] | Discount For Small Issue Size [Member]
Fair Value [Line Items]
Range	0.70%	0.70%
Municipal [Member] | Discounted cash flow [Member] | Level 3 [Member] | Minimum [Member] | Discount For Credit Stress [Member]
Fair Value [Line Items]
Range		0.90%
Municipal [Member] | Discounted cash flow [Member] | Level 3 [Member] | Minimum [Member] | Discount For Above Market Coupon [Member]
Fair Value [Line Items]
Range	0.30%	0.30%
Municipal [Member] | Discounted cash flow [Member] | Level 3 [Member] | Maximum [Member] | Discount For Small Issue Size [Member]
Fair Value [Line Items]
Range	6.80%	6.80%
Municipal [Member] | Discounted cash flow [Member] | Level 3 [Member] | Maximum [Member] | Discount For Credit Stress [Member]
Fair Value [Line Items]
Range		1.50%
Municipal [Member] | Discounted cash flow [Member] | Level 3 [Member] | Maximum [Member] | Discount For Above Market Coupon [Member]
Fair Value [Line Items]
Range	0.50%	0.50%
Municipal [Member] | Discounted cash flow [Member] | Level 3 [Member] | Weighted Average [Member] | Discount For Small Issue Size [Member]
Fair Value [Line Items]
Range	3.20%	3.30%
Municipal [Member] | Discounted cash flow [Member] | Level 3 [Member] | Weighted Average [Member] | Discount For Credit Stress [Member]
Fair Value [Line Items]
Range	1.30%	1.20%
Municipal [Member] | Discounted cash flow [Member] | Level 3 [Member] | Weighted Average [Member] | Discount For Above Market Coupon [Member]
Fair Value [Line Items]
Range	0.40%	0.40%
Corporate [Member]
Fair Value [Line Items]
Fair Value	$ 0.8	$ 0.9	3.9	4.2
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] | Discount For Small Issue Size [Member]
Fair Value [Line Items]
Range	2.50%	2.50%
Corporate [Member] | Discounted cash flow [Member] | Level 3 [Member] | Discount For Above Market Coupon [Member]
Fair Value [Line Items]
Range	0.30%	0.30%
Corporate [Member] | Discounted cash flow [Member] | Level 3 [Member] | Weighted Average [Member] | Discount For Small Issue Size [Member]
Fair Value [Line Items]
Range	2.50%	2.50%
Corporate [Member] | Discounted cash flow [Member] | Level 3 [Member] | Weighted Average [Member] | Discount For Above Market Coupon [Member]
Fair Value [Line Items]
Range	0.30%	0.30%
Commercial Mortgage Backed [Member]
Fair Value [Line Items]
Fair Value	$ 13.5	$ 14.2	$ 14.6	$ 15.0
Commercial Mortgage Backed [Member] | Discounted cash flow [Member] | Level 3 [Member]
Fair Value [Line Items]
Fair Value	$ 13.5	$ 14.2
Commercial Mortgage Backed [Member] | Discounted cash flow [Member] | Level 3 [Member] | Discount For Above Market Coupon [Member]
Fair Value [Line Items]
Range	0.50%	0.50%
Commercial Mortgage Backed [Member] | Discounted cash flow [Member] | Level 3 [Member] | Discount For Lease Structure [Member]
Fair Value [Line Items]
Range	0.30%	0.30%
Commercial Mortgage Backed [Member] | Discounted cash flow [Member] | Level 3 [Member] | Minimum [Member] | Discount For Small Issue Size [Member]
Fair Value [Line Items]
Range	1.90%	1.90%
Commercial Mortgage Backed [Member] | Discounted cash flow [Member] | Level 3 [Member] | Maximum [Member] | Discount For Small Issue Size [Member]
Fair Value [Line Items]
Range	3.10%	3.10%
Commercial Mortgage Backed [Member] | Discounted cash flow [Member] | Level 3 [Member] | Weighted Average [Member] | Discount For Small Issue Size [Member]
Fair Value [Line Items]
Range	2.70%	2.60%
Commercial Mortgage Backed [Member] | Discounted cash flow [Member] | Level 3 [Member] | Weighted Average [Member] | Discount For Above Market Coupon [Member]
Fair Value [Line Items]
Range	0.50%	0.50%
Commercial Mortgage Backed [Member] | Discounted cash flow [Member] | Level 3 [Member] | Weighted Average [Member] | Discount For Lease Structure [Member]
Fair Value [Line Items]
Range	0.30%	0.30%
Equity Securities [Member] | Market comparables [Member] | Level 3 [Member]
Fair Value [Line Items]
Fair Value	$ 1.1	$ 1.1
Fair value measurement market multiples	1.0	1.0
Equity Securities [Member] | Market comparables [Member] | Level 3 [Member] | Weighted Average [Member]
Fair Value [Line Items]
Fair value measurement market multiples	1.0	1.0
Other securities [Member] | Discounted cash flow [Member] | Level 3 [Member]
Fair Value [Line Items]
Fair Value	$ 3.6	$ 3.6
Range	18.00%	18.00%
Other securities [Member] | Discounted cash flow [Member] | Level 3 [Member] | Weighted Average [Member]
Fair Value [Line Items]
Range	18.00%	18.00%