Length of Time Fixed Maturity Securities had been in a Gross Unrealized Loss Position (Detail) - USD ($) $ in Millions		Mar. 31, 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		$ 6,713.6		$ 3,333.3
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		167.7		69.9
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer		14,933.8		15,553.4
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		2,683.1		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		26.2		21.8
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		0.6		0.4
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer		233.6		235.6
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		16.6		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		162.9		138.2
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		3.4		3.6
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer		1,907.1		1,922.2
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		471.7		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		81.2		264.6
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		1.7		5.5
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer		344.3		318.2
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		157.7		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		1,048.6		642.1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		32.9		19.4
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer		1,668.0		1,710.3
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		300.8		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]	539.5		206.6
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	[1]	5.2		0.4
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	[1]	261.0		275.3
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	[1]	17.5		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		4,851.2		2,060.0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		123.9		40.6
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer		10,516.0		11,087.9
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		1,718.6		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	[2]	0.0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		0.0	[2]	0.0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer		3.8	[2]	3.9
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		$ 0.2	[2]	$ 0.1

[1] 2Includes credit-tranched securities collateralized by loan obligations, auto loans, and other asset types

[2] 2Includes a de minimis gross unrealized loss for which the length of time has been for less than 12 months