Investments (Schedule of Fixed Maturities) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Investments in Fixed Maturities	$ 6,266.9	$ 6,284.5
Fair Value	6,573.1	6,424.2
Investments in Fixed Maturities
Debt Securities, Available-for-sale [Line Items]
Investments in Fixed Maturities	6,266.9	6,284.5
Gross unrealized gains	340.8	226.3
Gross unrealized loss	34.6	86.6
Fair Value	6,573.1	6,424.2
U.S. Government and Government Agencies and Authorities
Debt Securities, Available-for-sale [Line Items]
Investments in Fixed Maturities	845.9	865.9
Gross unrealized gains	20.2	14.8
Gross unrealized loss	7.7	15.0
Fair Value	858.4	865.7
States and Political Subdivisions
Debt Securities, Available-for-sale [Line Items]
Investments in Fixed Maturities	1,582.2	1,553.7
Gross unrealized gains	101.4	74.0
Gross unrealized loss	1.7	8.6
Fair Value	1,681.9	1,619.1
Foreign Governments
Debt Securities, Available-for-sale [Line Items]
Investments in Fixed Maturities	5.8	6.5
Gross unrealized gains	0.0	0.0
Gross unrealized loss	0.6	0.6
Fair Value	5.2	5.9
Bonds and Notes
Debt Securities, Available-for-sale [Line Items]
Investments in Fixed Maturities	3,281.5	3,307.8
Gross unrealized gains	216.4	135.1
Gross unrealized loss	17.8	49.1
Fair Value	3,480.1	3,393.8
Collateralized Loan Obligations
Debt Securities, Available-for-sale [Line Items]
Investments in Fixed Maturities	536.8	535.7
Gross unrealized gains	1.5	1.5
Gross unrealized loss	6.8	13.2
Fair Value	531.5	524.0
Other Mortgage- and Asset-backed
Debt Securities, Available-for-sale [Line Items]
Investments in Fixed Maturities	14.7	14.9
Gross unrealized gains	1.3	0.9
Gross unrealized loss	0.0	0.1
Fair Value	$ 16.0	$ 15.7

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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