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Amortized Cost and Fair Values of Securities by Security Type (Detail) - USD ($) $ in Millions	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Dec. 31, 2021
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	$ 38,224.3		$ 37,825.2
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	424.7
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	4,578.3
Debt Securities, Available-for-sale, Total	34,070.7		34,840.8
United States Government and Government Agencies and Authorities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	605.2		503.8
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	7.9		20.3
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	38.1		25.9
Debt Securities, Available-for-sale, Total	575.0		498.2
States, Municipalities, and Political Subdivisions
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	4,066.9		4,006.0
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	49.8		87.1
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	725.6		635.9
Debt Securities, Available-for-sale, Total	3,391.1		3,457.2
Foreign Governments
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	945.7		908.1
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	12.8		34.9
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	138.9		115.9
Debt Securities, Available-for-sale, Total	819.6		827.1
Public Utilities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	5,410.5		5,170.9
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	0.0	$ 0.0	0.0	$ 4.6	$ 4.1	$ 0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	67.7		141.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	517.0		355.0
Debt Securities, Available-for-sale, Total	4,961.2		4,956.9
Mortgage/Asset-backed Securities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	655.0		592.1
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	4.1		8.2
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	54.2		27.0
Debt Securities, Available-for-sale, Total	604.9		573.3
All Other Corporate Bonds
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	26,537.0		26,640.3
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	282.4		452.1
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	3,104.0		2,567.8
Debt Securities, Available-for-sale, Total	23,715.4		24,524.6
Redeemable Preferred Stocks
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	4.0		4.0
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	0.5		0.5
Debt Securities, Available-for-sale, Total	3.5		3.5
Fixed Maturity Securities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	38,224.3		37,825.2
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	424.7		743.6
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	4,578.3		3,728.0
Debt Securities, Available-for-sale, Total	$ 34,070.7		$ 34,840.8