Amortized Cost and Fair Values of Securities by Security Type (Detail) - USD ($) $ in Millions		Mar. 31, 2026	Dec. 31, 2025	Mar. 31, 2025	Dec. 31, 2024
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities		$ 35,530.8	$ 34,747.2
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities		0.0	5.9
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax		620.8	822.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax		2,850.8	2,506.7
Debt Securities, Available-for-sale, Total		33,300.8	33,056.6
United States Government and Government Agencies and Authorities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities		548.9	543.8
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		13.2	17.2
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax		17.2	16.0
Debt Securities, Available-for-sale, Total		544.9	545.0
States, Municipalities, and Political Subdivisions
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities		3,413.2	3,450.7
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		61.9	73.8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax		475.1	466.4
Debt Securities, Available-for-sale, Total		3,000.0	3,058.1
Foreign Governments
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities		989.4	1,008.8
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		14.9	18.8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax		159.4	157.3
Debt Securities, Available-for-sale, Total		844.9	870.3
Public Utilities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities		5,202.3	5,048.6
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		136.8	175.4
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax		333.7	290.1
Debt Securities, Available-for-sale, Total		5,005.4	4,933.9
Mortgage/Asset-backed Securities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	[1]	1,371.9	1,187.3
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	[1]	0.0	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	[1]	7.7	9.3
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	[1]	22.7	17.3
Debt Securities, Available-for-sale, Total	[1]	1,356.9	1,179.3
All Other Corporate Bonds
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities		23,997.1	23,500.0
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities		0.0	5.9	$ 3.8	$ 2.8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax		386.3	527.5
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax		1,842.5	1,559.5
Debt Securities, Available-for-sale, Total		22,540.9	22,462.1
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		0.2	0.1
Debt Securities, Available-for-sale, Total		$ 7.8	$ 7.9

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