Investments - Continuous Unrealized Loss (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
U.S. Government and Government Agencies and Authorities
Fair Value
Less Than 12 Months	$ 45.0	$ 10.5
12 Months or Longer	1.2	0.0
Total	46.2	10.5
Unrealized Losses
Less Than 12 Months	(0.7)	(0.1)
12 Months or Longer	(0.1)	0.0
Total	(0.8)	(0.1)
States and Political Subdivisions
Fair Value
Less Than 12 Months	468.1	23.3
12 Months or Longer	0.2	0.0
Total	468.3	23.3
Unrealized Losses
Less Than 12 Months	(11.8)	(0.2)
12 Months or Longer	0.0	0.0
Total	(11.8)	(0.2)
Foreign Governments
Fair Value
Less Than 12 Months	0.5	0.5
12 Months or Longer	2.7	2.6
Total	3.2	3.1
Unrealized Losses
Less Than 12 Months	(0.5)	(0.1)
12 Months or Longer	(0.6)	(1.0)
Total	(1.1)	(1.1)
Bonds and Notes
Fair Value
Less Than 12 Months	495.1	132.9
12 Months or Longer	26.5	46.1
Total	521.6	179.0
Unrealized Losses
Less Than 12 Months	(14.0)	(7.5)
12 Months or Longer	(1.8)	(3.1)
Total	(15.8)	(10.6)
Collateralized Loan Obligations
Fair Value
Less Than 12 Months	206.1	145.2
12 Months or Longer	234.9	371.4
Total	441.0	516.6
Unrealized Losses
Less Than 12 Months	(2.4)	(3.8)
12 Months or Longer	(2.3)	(15.9)
Total	(4.7)	(19.7)
Other Mortgage- and Asset-backed
Fair Value
Less Than 12 Months	63.2	6.3
12 Months or Longer	0.0	0.0
Total	63.2	6.3
Unrealized Losses
Less Than 12 Months	(0.7)	0.0
12 Months or Longer	0.0	0.0
Total	(0.7)	0.0
Investments in Fixed Maturities
Fair Value
Less Than 12 Months	1,278.0	318.7
12 Months or Longer	265.5	420.1
Total	1,543.5	738.8
Unrealized Losses
Less Than 12 Months	(30.1)	(11.7)
12 Months or Longer	(4.8)	(20.0)
Total	(34.9)	(31.7)
Investments in Fixed Maturities | Non-investment-grade
Unrealized Losses
Total	$ (7.7)	$ (23.7)