Investments - Aging of Unrealized Losses on Investments in Fixed Maturities (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Unrealized Losses
12 Months or Longer		$ 0.0
United States Government and Government Agencies and Authorities
Fair Value
Less Than 12 Months	$ 337.3	168.7
12 Months or Longer	126.5	1.2
Total	463.8	169.9
Unrealized Losses
Less Than 12 Months	(49.3)	(1.8)
12 Months or Longer	(36.5)	(0.1)
Total	(85.8)	(1.9)
States and Political Subdivisions
Fair Value
Less Than 12 Months	854.7	385.0
12 Months or Longer	276.8	1.5
Total	1,131.5	386.5
Unrealized Losses
Less Than 12 Months	(140.6)	(6.9)
12 Months or Longer	(97.7)	(0.1)
Total	(238.3)	(7.0)
Foreign Governments
Fair Value
Less Than 12 Months	0.1	2.2
12 Months or Longer	2.6	2.6
Total	2.7	4.8
Unrealized Losses
Less Than 12 Months	0.0	(0.6)
12 Months or Longer	(0.9)	(0.6)
Total	(0.9)	(1.2)
Corporate Bonds and Notes
Fair Value
Less Than 12 Months	2,730.6	596.8
12 Months or Longer	424.4	49.3
Total	3,155.0	646.1
Unrealized Losses
Less Than 12 Months	(373.9)	(13.1)
12 Months or Longer	(125.8)	(2.9)
Total	(499.7)	(16.0)
Redeemable Preferred Stocks
Fair Value
Less Than 12 Months	7.7	0.1
12 Months or Longer	0.0	0.0
Total	7.7	0.1
Available-for-Sale Securities, Continuous Unrealized Loss Position, Accumulated Loss		0.0
Unrealized Losses
Less Than 12 Months	(1.0)	0.0
12 Months or Longer	0.0
Total	(1.0)
Collateralized Loan Obligations
Fair Value
Less Than 12 Months	568.2	250.9
12 Months or Longer	373.9	192.6
Total	942.1	443.5
Unrealized Losses
Less Than 12 Months	(34.2)	(2.6)
12 Months or Longer	(26.6)	(2.2)
Total	(60.8)	(4.8)
Other Mortgage- and Asset-backed
Fair Value
Less Than 12 Months	205.4	100.1
12 Months or Longer	79.5	0.0
Total	284.9	100.1
Unrealized Losses
Less Than 12 Months	(28.9)	(1.8)
12 Months or Longer	(21.4)	0.0
Total	(50.3)	(1.8)
Investments in Fixed Maturities
Fair Value
Less Than 12 Months	4,704.0	1,503.8
12 Months or Longer	1,283.7	247.2
Total	5,987.7	1,751.0
Unrealized Losses
Less Than 12 Months	(627.9)	(26.8)
12 Months or Longer	(308.9)	(5.9)
Total	$ (936.8)	$ (32.7)

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