Fair Value (Schedule of Additional Information About Significant Unobservable Inputs Used in Fair Valuations of Level 3) (Details) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2016 USD ($)	Dec. 31, 2015 USD ($)	Jun. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Dec. 31, 2014 USD ($)
Fair Value [Line Items]
Fair Value	$ 57.8	$ 60.5	$ 60.8	$ 58.7	$ 60.8	$ 61.7
Municipal [Member] | Discounted cash flow [Member] | Level 3
Fair Value [Line Items]
Fair Value	$ 32.3	$ 34.4
Municipal [Member] | Discount For Small Issue Size | Discounted cash flow [Member] | Level 3 | Minimum
Fair Value [Line Items]
Range	0.70%	0.60%
Municipal [Member] | Discount For Small Issue Size | Discounted cash flow [Member] | Level 3 | Maximum
Fair Value [Line Items]
Range	6.80%	6.80%
Municipal [Member] | Discount For Small Issue Size | Discounted cash flow [Member] | Level 3 | Weighted Average
Fair Value [Line Items]
Range	3.30%	3.20%
Municipal [Member] | Discount For Above Market Coupon | Discounted cash flow [Member] | Level 3 | Minimum
Fair Value [Line Items]
Range	0.30%	0.30%
Municipal [Member] | Discount For Above Market Coupon | Discounted cash flow [Member] | Level 3 | Maximum
Fair Value [Line Items]
Range	1.00%	1.00%
Municipal [Member] | Discount For Above Market Coupon | Discounted cash flow [Member] | Level 3 | Weighted Average
Fair Value [Line Items]
Range	0.40%	0.40%
Municipal [Member] | Discount For Credit Stress | Discounted cash flow [Member] | Level 3 | Minimum
Fair Value [Line Items]
Range	0.90%	0.90%
Municipal [Member] | Discount For Credit Stress | Discounted cash flow [Member] | Level 3 | Maximum
Fair Value [Line Items]
Range	1.50%	1.50%
Municipal [Member] | Discount For Credit Stress | Discounted cash flow [Member] | Level 3 | Weighted Average
Fair Value [Line Items]
Range	1.20%	1.20%
Corporate [Member] | Discounted cash flow [Member] | Level 3
Fair Value [Line Items]
Fair Value	$ 4.1	$ 3.7
Corporate [Member] | Discount For Small Issue Size | Discounted cash flow [Member] | Level 3
Fair Value [Line Items]
Range		1.00%
Corporate [Member] | Discount For Small Issue Size | Discounted cash flow [Member] | Level 3 | Minimum
Fair Value [Line Items]
Range	2.00%
Corporate [Member] | Discount For Small Issue Size | Discounted cash flow [Member] | Level 3 | Maximum
Fair Value [Line Items]
Range	2.50%
Corporate [Member] | Discount For Small Issue Size | Discounted cash flow [Member] | Level 3 | Weighted Average
Fair Value [Line Items]
Range	2.10%	1.00%
Corporate [Member] | Discount For Above Market Coupon | Discounted cash flow [Member] | Level 3 | Minimum
Fair Value [Line Items]
Range	0.30%	0.30%
Corporate [Member] | Discount For Above Market Coupon | Discounted cash flow [Member] | Level 3 | Maximum
Fair Value [Line Items]
Range	0.80%	0.80%
Corporate [Member] | Discount For Above Market Coupon | Discounted cash flow [Member] | Level 3 | Weighted Average
Fair Value [Line Items]
Range	0.60%	0.60%
Corporate [Member] | Discount For Credit Stress | Discounted cash flow [Member] | Level 3
Fair Value [Line Items]
Range	1.00%	10.00%
Corporate [Member] | Discount For Credit Stress | Discounted cash flow [Member] | Level 3 | Weighted Average
Fair Value [Line Items]
Range	1.00%	10.00%
Commercial Mortgage Backed [Member] | Discounted cash flow [Member] | Level 3
Fair Value [Line Items]
Fair Value	$ 16.2	$ 17.0
Commercial Mortgage Backed [Member] | Discount For Small Issue Size | Discounted cash flow [Member] | Level 3 | Minimum
Fair Value [Line Items]
Range	1.50%	0.50%
Commercial Mortgage Backed [Member] | Discount For Small Issue Size | Discounted cash flow [Member] | Level 3 | Maximum
Fair Value [Line Items]
Range	3.10%	1.00%
Commercial Mortgage Backed [Member] | Discount For Small Issue Size | Discounted cash flow [Member] | Level 3 | Weighted Average
Fair Value [Line Items]
Range	2.60%	0.50%
Commercial Mortgage Backed [Member] | Discount For Above Market Coupon | Discounted cash flow [Member] | Level 3
Fair Value [Line Items]
Range	0.50%	0.50%
Commercial Mortgage Backed [Member] | Discount For Above Market Coupon | Discounted cash flow [Member] | Level 3 | Weighted Average
Fair Value [Line Items]
Range	0.50%	0.50%
Commercial Mortgage Backed [Member] | Discount For Lease Structure | Discounted cash flow [Member] | Level 3
Fair Value [Line Items]
Range	0.30%	0.30%
Commercial Mortgage Backed [Member] | Discount For Lease Structure | Discounted cash flow [Member] | Level 3 | Weighted Average
Fair Value [Line Items]
Range	0.30%	0.30%
Asset-backed Securities [Member] | Discounted cash flow [Member] | Level 3
Fair Value [Line Items]
Fair Value		$ 0.5
Asset-backed Securities [Member] | Discount For Small Issue Size | Discounted cash flow [Member] | Level 3
Fair Value [Line Items]
Range		0.70%
Asset-backed Securities [Member] | Discount For Small Issue Size | Discounted cash flow [Member] | Level 3 | Weighted Average
Fair Value [Line Items]
Range		0.70%
Equity Securities [Member] | Market comparables [Member] | Level 3
Fair Value [Line Items]
Fair Value	$ 1.1	$ 1.1
Equity Securities [Member] | Net Tangible Asset Market Multiples | Market comparables [Member] | Level 3
Fair Value [Line Items]
Fair value measurement market multiples	1.0	1.0
Equity Securities [Member] | Net Tangible Asset Market Multiples | Market comparables [Member] | Level 3 | Weighted Average
Fair Value [Line Items]
Fair value measurement market multiples	1.0	1.0
Other securities | Discounted cash flow [Member] | Level 3
Fair Value [Line Items]
Fair Value	$ 3.6	$ 3.6
Other securities | Discount Rate [Member] | Discounted cash flow [Member] | Level 3
Fair Value [Line Items]
Range	18.00%	18.00%
Other securities | Discount Rate [Member] | Discounted cash flow [Member] | Level 3 | Weighted Average
Fair Value [Line Items]
Range	18.00%	18.00%