Fair Value (Schedule of Additional Information About Significant Unobservable Inputs Used in Fair Valuations of Level 3) (Details) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)	Mar. 31, 2018 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Fair Value [Line Items]
Fair Value	$ 43.2	$ 46.6	$ 44.3	$ 51.3	$ 54.9	$ 55.8
Municipal [Member]
Fair Value [Line Items]
Fair Value	24.4	26.8	25.3	30.1	30.8	31.0
Municipal [Member] | Discounted cash flow [Member] | Level 3 [Member]
Fair Value [Line Items]
Fair Value	$ 24.4	$ 26.8
Municipal [Member] | Discounted cash flow [Member] | Level 3 [Member] | Measurement Input, Discount Rate [Member]
Fair Value [Line Items]
Credit stress	1.30%
Municipal [Member] | Discounted cash flow [Member] | Level 3 [Member] | Minimum [Member] | Measurement Input, Discount Rate [Member]
Fair Value [Line Items]
Small issue size	0.70%	0.70%
Credit stress		0.90%
Above-market coupon	0.30%	0.30%
Municipal [Member] | Discounted cash flow [Member] | Level 3 [Member] | Maximum [Member] | Measurement Input, Discount Rate [Member]
Fair Value [Line Items]
Small issue size	6.80%	6.80%
Credit stress		1.50%
Above-market coupon	0.50%	0.50%
Municipal [Member] | Discounted cash flow [Member] | Level 3 [Member] | Weighted Average [Member] | Measurement Input, Discount Rate [Member]
Fair Value [Line Items]
Small issue size	3.20%	3.30%
Credit stress	1.30%	1.20%
Above-market coupon	0.40%	0.40%
Corporate [Member]
Fair Value [Line Items]
Fair Value	$ 0.8	$ 0.9	0.8	1.0	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] | 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]
Fair Value [Line Items]
Fair Value	$ 13.3	$ 14.2	$ 13.5	$ 14.6	$ 14.6	$ 15.0
Commercial Mortgage Backed [Member] | Discounted cash flow [Member] | Level 3 [Member]
Fair Value [Line Items]
Fair Value	$ 13.3	$ 14.2
Commercial Mortgage Backed [Member] | Discounted cash flow [Member] | Level 3 [Member] | Measurement Input, Discount Rate [Member]
Fair Value [Line Items]
Above-market coupon	0.50%	0.50%
Lease structure	0.30%	0.30%
Commercial Mortgage Backed [Member] | Discounted cash flow [Member] | Level 3 [Member] | Minimum [Member] | Measurement Input, Discount Rate [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 Rate [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 Rate [Member]
Fair Value [Line Items]
Small issue size	2.70%	2.60%
Above-market coupon	0.50%	0.50%
Lease structure	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 | Discounted cash flow [Member] | Level 3 [Member]
Fair Value [Line Items]
Fair Value	$ 3.6	$ 3.6
Other securities | Discounted cash flow [Member] | Level 3 [Member] | Measurement Input, Discount Rate [Member]
Fair Value [Line Items]
Range	0.180	0.180
Other securities | Discounted cash flow [Member] | Level 3 [Member] | Weighted Average [Member] | Measurement Input, Discount Rate [Member]
Fair Value [Line Items]
Range	0.180	0.180