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Amortized Cost and Fair Values of Securities by Security Type (Detail) - USD ($) $ in Millions	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2021
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	$ 38,028.1		$ 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	810.6
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	3,464.6
Debt Securities, Available-for-sale, Total	35,374.1		34,840.8
United States Government and Government Agencies and Authorities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	570.5		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	20.6		20.3
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	22.0		25.9
Debt Securities, Available-for-sale, Total	569.1		498.2
States, Municipalities, and Political Subdivisions
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	3,961.4		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	113.9		87.1
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	537.7		635.9
Debt Securities, Available-for-sale, Total	3,537.6		3,457.2
Foreign Governments
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	986.6		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	18.5		34.9
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	134.4		115.9
Debt Securities, Available-for-sale, Total	870.7		827.1
Public Utilities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	5,372.6		5,170.9
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	0.0	$ 0.0	0.0	$ 4.1	$ 4.1	$ 0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	149.1		141.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	345.5		355.0
Debt Securities, Available-for-sale, Total	5,176.2		4,956.9
Mortgage/Asset-backed Securities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	576.7		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	6.4		8.2
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	27.0		27.0
Debt Securities, Available-for-sale, Total	556.1		573.3
All Other Corporate Bonds
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	26,556.3		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	502.1		452.1
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	2,397.5		2,567.8
Debt Securities, Available-for-sale, Total	24,660.9		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	38,028.1		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	810.6		743.6
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	3,464.6		3,728.0
Debt Securities, Available-for-sale, Total	$ 35,374.1		$ 34,840.8