Amortized Cost and Fair Values of Securities by Security Type (Detail) - USD ($) $ in Millions		Sep. 30, 2025		Jun. 30, 2025	Dec. 31, 2024		Sep. 30, 2024	Jun. 30, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities		$ 34,861.7			$ 38,269.9
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities		15.2			2.8		$ 2.9		$ 2.2
Gross Unrealized Gain		857.0
Gross Unrealized Loss		2,513.3
Debt Securities, Available-for-sale, Total		33,190.2			35,629.9
United States Government and Government Agencies and Authorities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities		542.7			544.6
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities		0.0			0.0
Gross Unrealized Gain		18.7			13.9
Gross Unrealized Loss		17.4			28.0
Debt Securities, Available-for-sale, Total		544.0			530.5
States, Municipalities, and Political Subdivisions
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities		3,473.8			3,795.6
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities		0.0			0.0
Gross Unrealized Gain		74.4			65.5
Gross Unrealized Loss		480.2			569.7
Debt Securities, Available-for-sale, Total		3,068.0			3,291.4
Foreign Governments
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities		1,044.4			912.1
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities		0.0			0.0
Gross Unrealized Gain		14.5			9.5
Gross Unrealized Loss		179.4			153.5
Debt Securities, Available-for-sale, Total		879.5			768.1
Public Utilities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities		4,990.2			5,525.0
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities		0.0			0.0
Gross Unrealized Gain		180.8			132.3
Gross Unrealized Loss		282.2			364.4
Debt Securities, Available-for-sale, Total		4,888.8			5,292.9
Mortgage/Asset-backed Securities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	[1]	1,128.9			949.4
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	[1]	0.0			0.0
Gross Unrealized Gain	[1]	10.0			5.0
Gross Unrealized Loss	[1]	17.3			37.2
Debt Securities, Available-for-sale, Total		1,121.6	[2]		917.2	[1]
All Other Corporate Bonds
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities		23,673.7			26,535.2
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities		15.2		$ 14.4	2.8		$ 2.9	$ 5.1
Gross Unrealized Gain		558.6			450.6
Gross Unrealized Loss		1,536.6			2,160.8
Debt Securities, Available-for-sale, Total		22,680.5			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		34,861.7			38,269.9
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities		15.2			2.8
Gross Unrealized Gain		857.0			676.8
Gross Unrealized Loss		$ 2,513.3			$ 3,314.0

[1] 1Includes 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