Investments - Continuous Unrealized Loss (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
U.S. Government and Government Agencies and Authorities
Fair Value
Less Than 12 Months	$ 19.2	$ 10.5
12 Months or Longer	0.0	0.0
Total	19.2	10.5
Unrealized Losses
Less Than 12 Months	(0.6)	(0.1)
12 Months or Longer	0.0	0.0
Total	(0.6)	(0.1)
States and Political Subdivisions
Fair Value
Less Than 12 Months	170.4	23.3
12 Months or Longer	3.4	0.0
Total	173.8	23.3
Unrealized Losses
Less Than 12 Months	(2.2)	(0.2)
12 Months or Longer	(0.1)	0.0
Total	(2.3)	(0.2)
Foreign Governments
Fair Value
Less Than 12 Months	0.2	0.5
12 Months or Longer	2.8	2.6
Total	3.0	3.1
Unrealized Losses
Less Than 12 Months	(0.1)	(0.1)
12 Months or Longer	(1.1)	(1.0)
Total	(1.2)	(1.1)
Bonds and Notes
Fair Value
Less Than 12 Months	337.4	132.9
12 Months or Longer	17.0	46.1
Total	354.4	179.0
Unrealized Losses
Less Than 12 Months	(8.8)	(7.5)
12 Months or Longer	(1.7)	(3.1)
Total	(10.5)	(10.6)
Collateralized Loan Obligations
Fair Value
Less Than 12 Months	162.6	145.2
12 Months or Longer	246.8	371.4
Total	409.4	516.6
Unrealized Losses
Less Than 12 Months	(0.9)	(3.8)
12 Months or Longer	(4.4)	(15.9)
Total	(5.3)	(19.7)
Other Mortgage- and Asset-backed
Fair Value
Less Than 12 Months	5.5	6.3
12 Months or Longer	5.0	0.0
Total	10.5	6.3
Unrealized Losses
Less Than 12 Months	(0.1)	0.0
12 Months or Longer	0.0	0.0
Total	(0.1)	0.0
Investments in Fixed Maturities
Fair Value
Less Than 12 Months	695.3	318.7
12 Months or Longer	275.0	420.1
Total	970.3	738.8
Unrealized Losses
Less Than 12 Months	(12.7)	(11.7)
12 Months or Longer	(7.3)	(20.0)
Total	(20.0)	(31.7)
Investments in Fixed Maturities | Non-investment-grade
Unrealized Losses
Total	$ (8.5)	$ (23.7)