Amortized Cost and Fair Values of Securities by Security Type (Detail) - USD ($) $ in Millions		Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities		$ 38,269.9	$ 38,410.6
Debt Securities, Available-for-sale, Allowance for Credit Loss		2.8	2.2
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax		676.8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax		3,314.0
Debt Securities, Available-for-sale, Total		35,629.9	36,833.9
United States Government and Government Agencies and Authorities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities		544.6	618.6
Debt Securities, Available-for-sale, Allowance for Credit Loss		0.0	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax		13.9	25.3
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax		28.0	19.1
Debt Securities, Available-for-sale, Total		530.5	624.8
States, Municipalities, and Political Subdivisions
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities		3,795.6	4,041.3
Debt Securities, Available-for-sale, Allowance for Credit Loss		0.0	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax		65.5	135.3
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax		569.7	498.2
Debt Securities, Available-for-sale, Total		3,291.4	3,678.4
Foreign Governments
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities		912.1	982.1
Debt Securities, Available-for-sale, Allowance for Credit Loss		0.0	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax		9.5	29.8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax		153.5	121.2
Debt Securities, Available-for-sale, Total		768.1	890.7
Public Utilities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities		5,525.0	5,398.2
Debt Securities, Available-for-sale, Allowance for Credit Loss		0.0	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax		132.3	217.1
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax		364.4	293.7
Debt Securities, Available-for-sale, Total		5,292.9	5,321.6
Mortgage/Asset-backed Securities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	[1]	949.4	658.0
Debt Securities, Available-for-sale, Allowance for Credit Loss	[1]	0.0	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	[1]	5.0	10.1
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	[1]	37.2	24.0
Debt Securities, Available-for-sale, Total	[1]	917.2	644.1
All Other Corporate Bonds
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities		26,535.2	26,708.4
Debt Securities, Available-for-sale, Allowance for Credit Loss		2.8	2.2	$ 0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax		450.6	771.8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax		2,160.8	1,807.3
Debt Securities, Available-for-sale, Total		24,822.2	25,670.7
Redeemable Preferred Stocks
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities		8.0	4.0
Debt Securities, Available-for-sale, Allowance for Credit Loss		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		0.4	0.4
Debt Securities, Available-for-sale, Total		7.6	3.6
Fixed Maturity Securities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities		38,269.9	38,410.6
Debt Securities, Available-for-sale, Allowance for Credit Loss		2.8	2.2
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax		676.8	1,189.4
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax		3,314.0	2,763.9
Debt Securities, Available-for-sale, Total		$ 35,629.9	$ 36,833.9

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