Length of Time Fixed Maturity Securities had been in a Gross Unrealized Loss Position (Detail) - USD ($) $ in Millions		Jun. 30, 2026	Dec. 31, 2025
Debt Securities, Available-for-sale, Unrealized Loss Position
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months		$ 7,035.9	$ 3,333.3
Gross Unrealized Loss, Less than 12 Months		122.7	69.9
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer		14,622.9	15,553.4
Gross Unrealized Loss, Greater than 12 Months		2,542.6	2,436.8
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		87.0	21.8
Gross Unrealized Loss, Less than 12 Months		1.3	0.4
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer		229.6	235.6
Gross Unrealized Loss, Greater than 12 Months		18.0	15.6
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		197.9	138.2
Gross Unrealized Loss, Less than 12 Months		3.6	3.6
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer		1,909.5	1,922.2
Gross Unrealized Loss, Greater than 12 Months		463.1	462.8
Foreign Governments
Debt Securities, Available-for-sale, Unrealized Loss Position
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months		256.3	264.6
Gross Unrealized Loss, Less than 12 Months		2.9	5.5
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer		342.9	318.2
Gross Unrealized Loss, Greater than 12 Months		160.5	151.8
Public Utilities
Debt Securities, Available-for-sale, Unrealized Loss Position
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months		992.7	642.1
Gross Unrealized Loss, Less than 12 Months		26.5	19.4
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer		1,655.4	1,710.3
Gross Unrealized Loss, Greater than 12 Months		284.6	270.7
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]	584.1	206.6
Gross Unrealized Loss, Less than 12 Months	[1]	4.5	0.4
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	[1]	251.7	275.3
Gross Unrealized Loss, Greater than 12 Months	[1]	20.4	16.9
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		4,913.9	2,060.0
Gross Unrealized Loss, Less than 12 Months		83.9	40.6
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer		10,230.0	11,087.9
Gross Unrealized Loss, Greater than 12 Months		1,595.8	1,518.9
Redeemable Preferred Stocks
Debt Securities, Available-for-sale, Unrealized Loss Position
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months		4.0	0.0
Gross Unrealized Loss, Less than 12 Months		0.0	0.0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer		3.8	3.9
Gross Unrealized Loss, Greater than 12 Months		$ 0.2	$ 0.1

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