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Amortized Cost and Fair Values of Securities by Security Type (Detail) - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024	Mar. 31, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	$ 37,860.2	$ 38,269.9
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	3.8	2.8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	796.1
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	2,901.1
Debt Securities, Available-for-sale, Total	35,751.4	35,629.9
United States Government and Government Agencies and Authorities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	537.4	544.6
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	0.0	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	16.9	13.9
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	20.6	28.0
Debt Securities, Available-for-sale, Total	533.7	530.5
States, Municipalities, and Political Subdivisions
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	3,682.7	3,795.6
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	0.0	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	73.8	65.5
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	498.7	569.7
Debt Securities, Available-for-sale, Total	3,257.8	3,291.4
Foreign Governments
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	960.7	912.1
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	0.0	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	13.7	9.5
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	164.1	153.5
Debt Securities, Available-for-sale, Total	810.3	768.1
Public Utilities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	5,442.0	5,525.0
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	0.0	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	167.5	132.3
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	322.3	364.4
Debt Securities, Available-for-sale, Total	5,287.2	5,292.9
Mortgage/Asset-backed Securities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	980.1	949.4
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	0.0	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	5.5	5.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	24.8	37.2
Debt Securities, Available-for-sale, Total	960.8	917.2
All Other Corporate Bonds
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	26,249.3	26,535.2
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	3.8	2.8	$ 2.2	$ 2.2
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	518.7	450.6
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	1,870.3	2,160.8
Debt Securities, Available-for-sale, Total	24,893.9	24,822.2
Redeemable Preferred Stocks
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	8.0	8.0
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	0.0	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	0.0	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0.3	0.4
Debt Securities, Available-for-sale, Total	7.7	7.6
Fixed Maturity Securities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	37,860.2	38,269.9
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	3.8	2.8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	796.1	676.8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	2,901.1	3,314.0
Debt Securities, Available-for-sale, Total	$ 35,751.4	$ 35,629.9