Investments (Schedule of Fixed Maturities) (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Investments in Fixed Maturities	$ 5,076.0	$ 5,021.6
Fair Value	5,260.3	5,382.7
Investments in Fixed Maturities
Debt Securities, Available-for-sale [Line Items]
Investments in Fixed Maturities	5,076.0	5,021.6
Gross unrealized gains	251.0	380.6
Gross unrealized loss	66.7	19.5
Fair Value	5,260.3	5,382.7
U.S. Government and Government Agencies and Authorities
Debt Securities, Available-for-sale [Line Items]
Investments in Fixed Maturities	656.4	542.7
Gross unrealized gains	12.9	19.6
Gross unrealized loss	19.9	6.2
Fair Value	649.4	556.1
States and Political Subdivisions
Debt Securities, Available-for-sale [Line Items]
Investments in Fixed Maturities	1,465.2	1,595.5
Gross unrealized gains	77.3	108.6
Gross unrealized loss	10.1	2.3
Fair Value	1,532.4	1,701.8
Foreign Governments
Debt Securities, Available-for-sale [Line Items]
Investments in Fixed Maturities	4.7	3.0
Gross unrealized gains	0.0	0.2
Gross unrealized loss	0.4	0.0
Fair Value	4.3	3.2
Bonds and Notes
Debt Securities, Available-for-sale [Line Items]
Investments in Fixed Maturities	2,717.7	2,745.8
Gross unrealized gains	157.1	245.8
Gross unrealized loss	35.4	11.0
Fair Value	2,839.4	2,980.6
Redeemable Preferred Stocks
Debt Securities, Available-for-sale [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
Debt Securities, Available-for-sale [Line Items]
Investments in Fixed Maturities	227.0	134.1
Gross unrealized gains	3.0	5.7
Gross unrealized loss	0.8	0.0
Fair Value	229.2	139.8
Other Mortgage- and Asset-backed
Debt Securities, Available-for-sale [Line Items]
Investments in Fixed Maturities	5.0	0.4
Gross unrealized gains	0.7	0.7
Gross unrealized loss	0.1	0.0
Fair Value	$ 5.6	$ 1.1