Amortized Cost and Fair Values of Securities by Security Type (Detail) - USD ($) $ in Millions		Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities		$ 34,747.2	$ 38,269.9
Debt Securities, Available-for-sale, Allowance for Credit Loss		5.9	2.8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax		822.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax		2,506.7
Debt Securities, Available-for-sale, Total		33,056.6	35,629.9
United States Government and Government Agencies and Authorities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities		543.8	544.6
Debt Securities, Available-for-sale, Allowance for Credit Loss		0.0	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax		17.2	13.9
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax		16.0	28.0
Debt Securities, Available-for-sale, Total		545.0	530.5
States, Municipalities, and Political Subdivisions
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities		3,450.7	3,795.6
Debt Securities, Available-for-sale, Allowance for Credit Loss		0.0	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax		73.8	65.5
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax		466.4	569.7
Debt Securities, Available-for-sale, Total		3,058.1	3,291.4
Foreign Governments
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities		1,008.8	912.1
Debt Securities, Available-for-sale, Allowance for Credit Loss		0.0	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax		18.8	9.5
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax		157.3	153.5
Debt Securities, Available-for-sale, Total		870.3	768.1
Public Utilities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities		5,048.6	5,525.0
Debt Securities, Available-for-sale, Allowance for Credit Loss		0.0	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax		175.4	132.3
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax		290.1	364.4
Debt Securities, Available-for-sale, Total		4,933.9	5,292.9
Mortgage/Asset-backed Securities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	[1]	1,187.3	949.4
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]	9.3	5.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	[1]	17.3	37.2
Debt Securities, Available-for-sale, Total	[1],[2]	1,179.3	917.2
All Other Corporate Bonds
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities		23,500.0	26,535.2
Debt Securities, Available-for-sale, Allowance for Credit Loss		5.9	2.8	$ 2.2
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax		527.5	450.6
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax		1,559.5	2,160.8
Debt Securities, Available-for-sale, Total		22,462.1	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 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		0.1	0.4
Debt Securities, Available-for-sale, Total		7.9	7.6
Fixed Maturity Securities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities		34,747.2	38,269.9
Debt Securities, Available-for-sale, Allowance for Credit Loss		5.9	2.8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax		822.0	676.8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax		2,506.7	3,314.0
Debt Securities, Available-for-sale, Total		$ 33,056.6	$ 35,629.9

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

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