Investments - Schedule of Fixed Maturities (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Investments in Fixed Maturities	$ 6,299.7	$ 6,284.5
Fixed maturities at fair value	6,883.6	6,424.2
Investments in Fixed Maturities
Debt Securities, Available-for-sale [Line Items]
Investments in Fixed Maturities	6,299.7	6,284.5
Gross unrealized gains	605.5	226.3
Gross unrealized loss	(21.6)	(86.6)
Fixed maturities at fair value	6,883.6	6,424.2
U.S. Government and Government Agencies and Authorities
Debt Securities, Available-for-sale [Line Items]
Investments in Fixed Maturities	797.1	865.9
Gross unrealized gains	42.1	14.8
Gross unrealized loss	(0.3)	(15.0)
Fixed maturities at fair value	838.9	865.7
States and Political Subdivisions
Debt Securities, Available-for-sale [Line Items]
Investments in Fixed Maturities	1,397.2	1,553.7
Gross unrealized gains	148.5	74.0
Gross unrealized loss	(0.6)	(8.6)
Fixed maturities at fair value	1,545.1	1,619.1
Foreign Governments
Debt Securities, Available-for-sale [Line Items]
Investments in Fixed Maturities	15.8	6.5
Gross unrealized gains	1.1	0.0
Gross unrealized loss	(2.5)	(0.6)
Fixed maturities at fair value	14.4	5.9
Bonds and Notes
Debt Securities, Available-for-sale [Line Items]
Investments in Fixed Maturities	3,506.5	3,307.8
Gross unrealized gains	410.4	135.1
Gross unrealized loss	(9.8)	(49.1)
Fixed maturities at fair value	3,907.1	3,393.8
Collateralized Loan Obligations
Debt Securities, Available-for-sale [Line Items]
Investments in Fixed Maturities	543.7	535.7
Gross unrealized gains	1.2	1.5
Gross unrealized loss	(8.4)	(13.2)
Fixed maturities at fair value	536.5	524.0
Other Mortgage- and Asset-backed
Debt Securities, Available-for-sale [Line Items]
Investments in Fixed Maturities	39.4	14.9
Gross unrealized gains	2.2	0.9
Gross unrealized loss	0.0	(0.1)
Fixed maturities at fair value	$ 41.6	$ 15.7