Amortized Cost and Fair Values of Securities by Security Type (Detail) - USD ($) $ in Millions		Sep. 30, 2026	Jun. 30, 2026	Dec. 31, 2025	Sep. 30, 2025	Jun. 30, 2025	Dec. 31, 2024
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities		$ 36,218.2		$ 34,747.2
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities		0.0		5.9
Gross Unrealized Gain		227.9		822.0
Gross Unrealized Loss		3,451.6		2,506.7
Debt Securities, Available-for-sale, Total		32,994.5		33,056.6
United States Government and Government Agencies and Authorities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities		840.4		543.8
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities		0.0		0.0
Gross Unrealized Gain		3.5		17.2
Gross Unrealized Loss		49.3		16.0
Debt Securities, Available-for-sale, Total		794.6		545.0
States, Municipalities, and Political Subdivisions
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities		3,253.0		3,450.7
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities		0.0		0.0
Gross Unrealized Gain		22.0		73.8
Gross Unrealized Loss		568.6		466.4
Debt Securities, Available-for-sale, Total		2,706.4		3,058.1
Foreign Governments
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities		1,023.4		1,008.8
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities		0.0		0.0
Gross Unrealized Gain		2.4		18.8
Gross Unrealized Loss		190.6		157.3
Debt Securities, Available-for-sale, Total		835.2		870.3
Public Utilities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities		5,215.1		5,048.6
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities		0.0		0.0
Gross Unrealized Gain		43.3		175.4
Gross Unrealized Loss		422.4		290.1
Debt Securities, Available-for-sale, Total		4,836.0		4,933.9
Mortgage/Asset-backed Securities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	[1]	1,680.9		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]	3.2		9.3
Gross Unrealized Loss	[1]	37.5		17.3
Debt Securities, Available-for-sale, Total	[1],[2]	1,646.6		1,179.3
All Other Corporate Bonds
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities		24,197.4		23,500.0
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities		0.0	$ 0.0	5.9	$ 15.2	$ 14.4	$ 2.8
Gross Unrealized Gain		153.5		527.5
Gross Unrealized Loss		2,182.9		1,559.5
Debt Securities, Available-for-sale, Total		22,168.0		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		0.3		0.1
Debt Securities, Available-for-sale, Total		$ 7.7		$ 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.