Amortized Cost and Fair Values of Securities by Security Type (Detail) - USD ($) $ in Millions		Jun. 30, 2026	Mar. 31, 2026	Dec. 31, 2025	Jun. 30, 2025	Mar. 31, 2025	Dec. 31, 2024
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities		$ 36,495.5		$ 34,747.2
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities		0.0		5.9
Gross Unrealized Gain		613.4		822.0
Gross Unrealized Loss		2,665.3		2,506.7
Debt Securities, Available-for-sale, Total		34,443.6		33,056.6
United States Government and Government Agencies and Authorities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities		635.2		543.8
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities		0.0		0.0
Gross Unrealized Gain		10.8		17.2
Gross Unrealized Loss		19.3		16.0
Debt Securities, Available-for-sale, Total		626.7		545.0
States, Municipalities, and Political Subdivisions
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities		3,365.3		3,450.7
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities		0.0		0.0
Gross Unrealized Gain		59.9		73.8
Gross Unrealized Loss		466.7		466.4
Debt Securities, Available-for-sale, Total		2,958.5		3,058.1
Foreign Governments
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities		1,014.8		1,008.8
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities		0.0		0.0
Gross Unrealized Gain		13.4		18.8
Gross Unrealized Loss		163.4		157.3
Debt Securities, Available-for-sale, Total		864.8		870.3
Public Utilities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities		5,222.1		5,048.6
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities		0.0		0.0
Gross Unrealized Gain		129.8		175.4
Gross Unrealized Loss		311.1		290.1
Debt Securities, Available-for-sale, Total		5,040.8		4,933.9
Mortgage/Asset-backed Securities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	[1]	1,497.3		1,187.3
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	[1]	0.0		0.0
Gross Unrealized Gain	[1]	7.1		9.3
Gross Unrealized Loss	[1]	24.9		17.3
Debt Securities, Available-for-sale, Total	[1],[2]	1,479.5		1,179.3
All Other Corporate Bonds
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities		24,752.8		23,500.0
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities		0.0	$ 0.0	5.9	$ 14.4	$ 3.8	$ 2.8
Gross Unrealized Gain		392.4		527.5
Gross Unrealized Loss		1,679.7		1,559.5
Debt Securities, Available-for-sale, Total		23,465.5		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
Gross Unrealized Gain		0.0		0.0
Gross Unrealized Loss		0.2		0.1
Debt Securities, Available-for-sale, Total		$ 7.8		$ 7.9

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

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