Length of Time Fixed Maturity Securities had been in a Gross Unrealized Loss Position (Detail) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
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	$ 118.8	$ 246.6
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0.7	22.6
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	197.3	12.2
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	18.4	3.3
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	128.0	1,920.1
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	4.0	476.1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	2,035.1	346.6
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	494.2	159.8
Foreign Governments
Debt Securities, Available-for-sale, Unrealized Loss Position
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	149.9	160.1
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	3.3	47.9
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	312.9	176.9
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	117.9	68.0
Public Utilities
Debt Securities, Available-for-sale, Unrealized Loss Position
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	373.7	2,242.2
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	10.4	252.0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	1,720.6	255.2
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	283.3	103.0
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	60.3	386.6
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	2.5	27.0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	316.7	0.1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	21.5	0.0
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	1,483.8	15,865.6
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	26.8	1,799.7
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	14,215.2	2,194.1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	1,780.5	768.1
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	3.5
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0.0	0.5
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	3.6	0.0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0.4	0.0
Fixed Maturity Securities
Debt Securities, Available-for-sale, Unrealized Loss Position
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	2,314.5	20,824.7
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	47.7	2,625.8
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	18,801.4	2,985.1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	$ 2,716.2	$ 1,102.2