Length of Time Fixed Maturity Securities had been in a Gross Unrealized Loss Position (Detail) - USD ($) $ in Millions		Jun. 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		$ 47.3	$ 43.7
Gross Unrealized Loss, Less than 12 Months		2.5	4.1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer		205.0	201.3
Gross Unrealized Loss, Greater than 12 Months		17.5	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		303.1	425.8
Gross Unrealized Loss, Less than 12 Months		15.7	15.3
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer		1,814.7	1,926.2
Gross Unrealized Loss, Greater than 12 Months		518.2	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		122.7	171.9
Gross Unrealized Loss, Less than 12 Months		10.9	10.6
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer		301.0	266.3
Gross Unrealized Loss, Greater than 12 Months		162.2	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		870.5	1,281.7
Gross Unrealized Loss, Less than 12 Months		41.6	48.4
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer		1,512.8	1,549.5
Gross Unrealized Loss, Greater than 12 Months		280.6	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]	277.1	199.9
Gross Unrealized Loss, Less than 12 Months	[1]	3.9	8.9
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	[1]	262.9	285.9
Gross Unrealized Loss, Greater than 12 Months	[1]	20.6	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		3,708.5	4,904.4
Gross Unrealized Loss, Less than 12 Months		141.3	182.5
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer		11,090.5	12,209.3
Gross Unrealized Loss, Greater than 12 Months		1,641.4	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		5,329.2	7,031.3
Gross Unrealized Loss, Less than 12 Months		215.9	269.9
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer		15,190.6	16,442.2
Gross Unrealized Loss, Greater than 12 Months		$ 2,640.7	$ 3,044.1

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