Investments - Aging of Unrealized Losses on Investments in Fixed Maturities (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
United States Government and Government Agencies and Authorities
Fair Value
Less Than 12 Months	$ 168.7	$ 10.5
12 Months or Longer	1.2	0.0
Total	169.9	10.5
Unrealized Losses
Less Than 12 Months	(1.8)	(0.1)
12 Months or Longer	(0.1)	0.0
Total	(1.9)	(0.1)
States and Political Subdivisions
Fair Value
Less Than 12 Months	385.0	23.3
12 Months or Longer	1.5	0.0
Total	386.5	23.3
Unrealized Losses
Less Than 12 Months	(6.9)	(0.2)
12 Months or Longer	(0.1)	0.0
Total	(7.0)	(0.2)
Foreign Governments
Fair Value
Less Than 12 Months	2.2	0.5
12 Months or Longer	2.6	2.6
Total	4.8	3.1
Unrealized Losses
Less Than 12 Months	(0.6)	(0.1)
12 Months or Longer	(0.6)	(1.0)
Total	(1.2)	(1.1)
Corporate Bonds and Notes
Fair Value
Less Than 12 Months	596.8	132.9
12 Months or Longer	49.3	46.1
Total	646.1	179.0
Unrealized Losses
Less Than 12 Months	(13.1)	(7.5)
12 Months or Longer	(2.9)	(3.1)
Total	(16.0)	(10.6)
Redeemable Preferred Stocks
Fair Value
Less Than 12 Months	0.1
12 Months or Longer	0.0
Total	0.1
Unrealized Losses
Less Than 12 Months	0.0
12 Months or Longer	0.0
Total	0.0
Collateralized Loan Obligations
Fair Value
Less Than 12 Months	250.9	145.2
12 Months or Longer	192.6	371.4
Total	443.5	516.6
Unrealized Losses
Less Than 12 Months	(2.6)	(3.8)
12 Months or Longer	(2.2)	(15.9)
Total	(4.8)	(19.7)
Other Mortgage- and Asset-backed
Fair Value
Less Than 12 Months	100.1	6.3
12 Months or Longer	0.0	0.0
Total	100.1	6.3
Unrealized Losses
Less Than 12 Months	(1.8)	0.0
12 Months or Longer	0.0	0.0
Total	(1.8)	0.0
Investments in Fixed Maturities
Fair Value
Less Than 12 Months	1,503.8	318.7
12 Months or Longer	247.2	420.1
Total	1,751.0	738.8
Unrealized Losses
Less Than 12 Months	(26.8)	(11.7)
12 Months or Longer	(5.9)	(20.0)
Total	$ (32.7)	$ (31.7)