Amortized Cost and Fair Values of Securities by Security Type (Detail) - USD ($) $ in Millions		Jun. 30, 2025	Mar. 31, 2025	Dec. 31, 2024	Jun. 30, 2024	Mar. 31, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities		$ 37,983.9		$ 38,269.9
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities		14.4		2.8	$ 5.1		$ 2.2
Gross Unrealized Gain		853.1
Gross Unrealized Loss		2,856.6
Debt Securities, Available-for-sale, Total		35,966.0		35,629.9
United States Government and Government Agencies and Authorities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities		540.0		544.6
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities		0.0		0.0
Gross Unrealized Gain		17.8		13.9
Gross Unrealized Loss		20.0		28.0
Debt Securities, Available-for-sale, Total		537.8		530.5
States, Municipalities, and Political Subdivisions
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities		3,651.0		3,795.6
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities		0.0		0.0
Gross Unrealized Gain		66.6		65.5
Gross Unrealized Loss		533.9		569.7
Debt Securities, Available-for-sale, Total		3,183.7		3,291.4
Foreign Governments
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities		1,032.3		912.1
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities		0.0		0.0
Gross Unrealized Gain		14.0		9.5
Gross Unrealized Loss		173.1		153.5
Debt Securities, Available-for-sale, Total		873.2		768.1
Public Utilities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities		5,468.7		5,525.0
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities		0.0		0.0
Gross Unrealized Gain		180.2		132.3
Gross Unrealized Loss		322.2		364.4
Debt Securities, Available-for-sale, Total		5,326.7		5,292.9
Mortgage/Asset-backed Securities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	[1]	1,110.8		949.4
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	[1]	0.0		0.0
Gross Unrealized Gain	[1]	6.9		5.0
Gross Unrealized Loss	[1]	24.5		37.2
Debt Securities, Available-for-sale, Total	[1]	1,093.2		917.2
All Other Corporate Bonds
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities		26,173.1		26,535.2
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities		14.4	$ 3.8	2.8	$ 5.1	$ 2.2
Gross Unrealized Gain		567.6		450.6
Gross Unrealized Loss		1,782.7		2,160.8
Debt Securities, Available-for-sale, Total		24,943.6		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 losses for Fixed Maturity Securities		0.0		0.0
Gross Unrealized Gain		0.0		0.0
Gross Unrealized Loss		0.2		0.4
Debt Securities, Available-for-sale, Total		7.8		7.6
Fixed Maturity Securities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities		37,983.9		38,269.9
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities		14.4		2.8
Gross Unrealized Gain		853.1		676.8
Gross Unrealized Loss		$ 2,856.6		$ 3,314.0

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