Length of Time Fixed Maturity Securities had been in a Gross Unrealized Loss Position (Detail) - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023
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	$ 101.7	$ 118.8
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	2.4	0.7
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	241.8	197.3
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	24.4	18.4
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	296.3	128.0
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	7.1	4.0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	2,035.6	2,035.1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	539.5	494.2
Foreign Governments
Debt Securities, Available-for-sale, Unrealized Loss Position
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	120.9	149.9
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	2.5	3.3
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	315.6	312.9
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	135.1	117.9
Public Utilities
Debt Securities, Available-for-sale, Unrealized Loss Position
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	1,159.9	373.7
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	31.1	10.4
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	1,824.2	1,720.6
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	341.7	283.3
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	187.8	60.3
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	4.6	2.5
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	348.9	316.7
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	32.6	21.5
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	2,972.9	1,483.8
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	95.4	26.8
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	14,162.7	14,215.2
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	2,103.0	1,780.5
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	0.0
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0.0	0.0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	3.6	3.6
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0.4	0.4
Fixed Maturity Securities
Debt Securities, Available-for-sale, Unrealized Loss Position
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	4,839.5	2,314.5
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	143.1	47.7
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	18,932.4	18,801.4
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	$ 3,176.7	$ 2,716.2