Length of Time Fixed Maturity Securities had been in a Gross Unrealized Loss Position (Detail) - USD ($) $ in Millions		Dec. 31, 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		$ 21.8	$ 43.7
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		0.4	4.1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer		235.6	201.3
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		15.6	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		138.2	425.8
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		3.6	15.3
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer		1,922.2	1,926.2
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		462.8	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		264.6	171.9
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		5.5	10.6
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer		318.2	266.3
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		151.8	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		642.1	1,281.7
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		19.4	48.4
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer		1,710.3	1,549.5
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		270.7	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]	206.6	199.9
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	[1]	0.4	8.9
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	[1]	275.3	285.9
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	[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		2,060.0	4,904.4
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		40.6	182.5
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer		11,087.9	12,209.3
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		1,518.9	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
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		0.0	0.1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer		3.9	3.7
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		0.1	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		3,333.3	7,031.3
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		69.9	269.9
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer		15,553.4	16,442.2
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		$ 2,436.8	$ 3,044.1

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