Length of Time Fixed Maturity Securities had been in a Gross Unrealized Loss Position (Detail) - USD ($) $ in Millions		Sep. 30, 2025	Dec. 31, 2024
United States Government and Government Agencies and Authorities
Debt Securities, Available-for-sale, Unrealized Loss Position
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months		$ 22.4	$ 43.7
Gross Unrealized Loss, Less than 12 Months		0.3	4.1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer		233.6	201.3
Gross Unrealized Loss, Greater than 12 Months		17.1	23.9
States, Municipalities, and Political Subdivisions
Debt Securities, Available-for-sale, Unrealized Loss Position
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months		168.2	425.8
Gross Unrealized Loss, Less than 12 Months		5.8	15.3
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer		1,902.4	1,926.2
Gross Unrealized Loss, Greater than 12 Months		474.4	554.4
Foreign Governments
Debt Securities, Available-for-sale, Unrealized Loss Position
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months		202.5	171.9
Gross Unrealized Loss, Less than 12 Months		7.9	10.6
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer		324.1	266.3
Gross Unrealized Loss, Greater than 12 Months		171.5	142.9
Public Utilities
Debt Securities, Available-for-sale, Unrealized Loss Position
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months		618.8	1,281.7
Gross Unrealized Loss, Less than 12 Months		20.0	48.4
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer		1,540.9	1,549.5
Gross Unrealized Loss, Greater than 12 Months		262.2	316.0
Mortgage/Asset-backed Securities
Debt Securities, Available-for-sale, Unrealized Loss Position
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	[1]	56.8	199.9
Gross Unrealized Loss, Less than 12 Months	[1]	0.4	8.9
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	[1]	281.7	285.9
Gross Unrealized Loss, Greater than 12 Months	[1]	16.9	28.3
All Other Corporate Bonds
Debt Securities, Available-for-sale, Unrealized Loss Position
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months		1,776.7	4,904.4
Gross Unrealized Loss, Less than 12 Months		47.9	182.5
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer		11,260.3	12,209.3
Gross Unrealized Loss, Greater than 12 Months		1,488.7	1,978.3
Redeemable Preferred Stocks
Debt Securities, Available-for-sale, Unrealized Loss Position
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months		0.0	3.9
Gross Unrealized Loss, Less than 12 Months		0.0	0.1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer		3.7	3.7
Gross Unrealized Loss, Greater than 12 Months		0.2	0.3
Fixed Maturity Securities
Debt Securities, Available-for-sale, Unrealized Loss Position
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months		2,845.4	7,031.3
Gross Unrealized Loss, Less than 12 Months		82.3	269.9
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer		15,546.7	16,442.2
Gross Unrealized Loss, Greater than 12 Months		$ 2,431.0	$ 3,044.1

[1] 1Includes credit-tranched securities collateralized by loan obligations, auto loans, and other asset types