Investments - Aging of Unrealized Losses on Investments in Fixed Maturities (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Unrealized Losses
12 Months or Longer		$ (0.8)
United States Government and Government Agencies and Authorities
Fair Value
Less Than 12 Months	$ 41.7	52.0
12 Months or Longer	383.6	401.6
Total	425.3	453.6
Unrealized Losses
Less Than 12 Months	(0.5)	(0.8)
12 Months or Longer	(101.9)	(83.7)
Total	(102.4)	(84.5)
States and Political Subdivisions
Fair Value
Less Than 12 Months	242.7	112.9
12 Months or Longer	933.4	928.3
Total	1,176.1	1,041.2
Unrealized Losses
Less Than 12 Months	(10.3)	(2.3)
12 Months or Longer	(215.1)	(187.5)
Total	(225.4)	(189.8)
Foreign Governments
Fair Value
Less Than 12 Months	0.0	0.0
12 Months or Longer	1.4	1.9
Total	1.4	1.9
Unrealized Losses
Less Than 12 Months	0.0	0.0
12 Months or Longer	(0.2)	(0.6)
Total	(0.2)	(0.6)
Bonds and Notes
Fair Value
Less Than 12 Months	674.3	198.4
12 Months or Longer	2,605.7	2,813.0
Total	3,280.0	3,011.4
Unrealized Losses
Less Than 12 Months	(40.9)	(5.5)
12 Months or Longer	(477.9)	(378.3)
Total	(518.8)	(383.8)
Redeemable Preferred Stocks
Fair Value
Less Than 12 Months	2.0	0.0
12 Months or Longer	6.6	7.9
Total	8.6	7.9
Unrealized Losses
Less Than 12 Months	0.0	0.0
12 Months or Longer	(1.0)
Total	(1.0)	(0.8)
Collateralized Loan Obligations
Fair Value
Less Than 12 Months	34.2	38.8
12 Months or Longer	89.5	747.7
Total	123.7	786.5
Unrealized Losses
Less Than 12 Months	(0.1)	(0.4)
12 Months or Longer	(7.1)	(24.1)
Total	(7.2)	(24.5)
Other Mortgage- and Asset-backed
Fair Value
Less Than 12 Months	12.0	15.7
12 Months or Longer	261.7	287.3
Total	273.7	303.0
Unrealized Losses
Less Than 12 Months	(0.1)	(0.1)
12 Months or Longer	(34.4)	(46.2)
Total	(34.5)	(46.3)
Investments in Fixed Maturities
Fair Value
Less Than 12 Months	1,006.9	417.8
12 Months or Longer	4,281.9	5,187.7
Total	5,288.8	5,605.5
Unrealized Losses
Less Than 12 Months	(51.9)	(9.1)
12 Months or Longer	(837.6)	(721.2)
Total	$ (889.5)	$ (730.3)