Investments (Schedule of Fixed Maturities) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Investments in Fixed Maturities	$ 5,060.0	$ 5,021.6
Fair Value	5,301.2	5,382.7
Investments in Fixed Maturities
Schedule of Available-for-sale Securities [Line Items]
Investments in Fixed Maturities	5,060.0	5,021.6
Gross unrealized gains	289.2	380.6
Gross unrealized loss	48.0	19.5
Fair Value	5,301.2	5,382.7
U.S. Government and Government Agencies and Authorities
Schedule of Available-for-sale Securities [Line Items]
Investments in Fixed Maturities	663.2	542.7
Gross unrealized gains	14.5	19.6
Gross unrealized loss	15.9	6.2
Fair Value	661.8	556.1
States and Political Subdivisions
Schedule of Available-for-sale Securities [Line Items]
Investments in Fixed Maturities	1,461.7	1,595.5
Gross unrealized gains	83.3	108.6
Gross unrealized loss	8.3	2.3
Fair Value	1,536.7	1,701.8
Foreign Governments
Schedule of Available-for-sale Securities [Line Items]
Investments in Fixed Maturities	3.1	3.0
Gross unrealized gains	0.1	0.2
Gross unrealized loss	0.0	0.0
Fair Value	3.2	3.2
Bonds and Notes
Schedule of Available-for-sale Securities [Line Items]
Investments in Fixed Maturities	2,712.7	2,745.8
Gross unrealized gains	185.8	245.8
Gross unrealized loss	23.7	11.0
Fair Value	2,874.8	2,980.6
Redeemable Preferred Stocks
Schedule of Available-for-sale Securities [Line Items]
Investments in Fixed Maturities		0.1
Gross unrealized gains		0.0
Gross unrealized loss		0.0
Fair Value		0.1
Collateralized Loan Obligations
Schedule of Available-for-sale Securities [Line Items]
Investments in Fixed Maturities	214.3	134.1
Gross unrealized gains	4.9	5.7
Gross unrealized loss	0.1	0.0
Fair Value	219.1	139.8
Other Mortgage- and Asset-backed
Schedule of Available-for-sale Securities [Line Items]
Investments in Fixed Maturities	5.0	0.4
Gross unrealized gains	0.6	0.7
Gross unrealized loss	0.0	0.0
Fair Value	$ 5.6	$ 1.1