Fair Value (Schedule of Additional Information About Significant Unobservable Inputs Used in Fair Valuations of Level 3) (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016	Jun. 30, 2017	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2015
Fair Value [Line Items]
Fair Value	$ 48.6	$ 55.8	$ 51.3	$ 57.8	$ 60.8	$ 60.5
Municipal [Member]
Fair Value [Line Items]
Fair Value	28.1	31.0	30.1	32.3	34.4	34.4
Municipal [Member] | Discounted cash flow [Member] | Level 3 [Member]
Fair Value [Line Items]
Fair Value	$ 28.1	$ 31.0
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%	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%	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	3.40%	3.30%
Municipal [Member] | Discounted cash flow [Member] | Level 3 [Member] | Weighted Average [Member] | Discount For Credit Stress [Member]
Fair Value [Line Items]
Range	1.20%	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	$ 0.9	$ 4.2	1.0	4.3	4.4	3.7
Corporate [Member] | Discounted cash flow [Member] | Level 3 [Member]
Fair Value [Line Items]
Fair Value	$ 0.9	$ 4.0
Corporate [Member] | Discounted cash flow [Member] | Level 3 [Member] | Discount For Small Issue Size [Member]
Fair Value [Line Items]
Range	2.50%
Corporate [Member] | Discounted cash flow [Member] | Level 3 [Member] | Discount For Credit Stress [Member]
Fair Value [Line Items]
Range		1.00%
Corporate [Member] | Discounted cash flow [Member] | Level 3 [Member] | Discount For Above Market Coupon [Member]
Fair Value [Line Items]
Range	0.30%
Corporate [Member] | Discounted cash flow [Member] | Level 3 [Member] | Minimum [Member] | Discount For Small Issue Size [Member]
Fair Value [Line Items]
Range		2.00%
Corporate [Member] | Discounted cash flow [Member] | Level 3 [Member] | Minimum [Member] | Discount For Above Market Coupon [Member]
Fair Value [Line Items]
Range		0.30%
Corporate [Member] | Discounted cash flow [Member] | Level 3 [Member] | Maximum [Member] | Discount For Small Issue Size [Member]
Fair Value [Line Items]
Range		2.50%
Corporate [Member] | Discounted cash flow [Member] | Level 3 [Member] | Maximum [Member] | Discount For Above Market Coupon [Member]
Fair Value [Line Items]
Range		0.80%
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%	2.10%
Corporate [Member] | Discounted cash flow [Member] | Level 3 [Member] | Weighted Average [Member] | Discount For Credit Stress [Member]
Fair Value [Line Items]
Range		1.00%
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%	0.60%
Commercial Mortgage Backed [Member]
Fair Value [Line Items]
Fair Value	$ 14.4	$ 15.0	$ 14.6	$ 16.2	$ 16.7	$ 17.0
Commercial Mortgage Backed [Member] | Discounted cash flow [Member] | Level 3 [Member]
Fair Value [Line Items]
Fair Value	$ 14.4	$ 15.0
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	2.60%	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
Equity Securities [Member] | Market comparables [Member] | Level 3 [Member] | Net Tangible Asset Market Multiples [Member]
Fair Value [Line Items]
Fair value measurement market multiples	1.0	1.0
Equity Securities [Member] | Market comparables [Member] | Level 3 [Member] | Weighted Average [Member] | Net Tangible Asset Market Multiples [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	$ 4.1	$ 4.1
Other securities [Member] | Discounted cash flow [Member] | Level 3 [Member] | Discount Rate [Member]
Fair Value [Line Items]
Range	18.00%	18.00%
Other securities [Member] | Discounted cash flow [Member] | Level 3 [Member] | Weighted Average [Member] | Discount Rate [Member]
Fair Value [Line Items]
Range	18.00%	18.00%