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Amortized Cost and Fair Values of Securities by Security Type (Detail) - USD ($) $ in Millions	Jun. 30, 2024	Mar. 31, 2024	Dec. 31, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	$ 38,554.3		$ 38,410.6
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	5.1		2.2	$ 0.0	$ 0.0	$ 0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	723.4
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	3,319.8
Debt Securities, Available-for-sale, Total	35,952.8		36,833.9
United States Government and Government Agencies and Authorities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	572.7		618.6
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	13.1		25.3
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	26.8		19.1
Debt Securities, Available-for-sale, Total	559.0		624.8
States, Municipalities, and Political Subdivisions
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	3,915.4		4,041.3
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	80.5		135.3
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	546.6		498.2
Debt Securities, Available-for-sale, Total	3,449.3		3,678.4
Foreign Governments
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	952.9		982.1
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	14.6		29.8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	137.6		121.2
Debt Securities, Available-for-sale, Total	829.9		890.7
Public Utilities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	5,543.1		5,398.2
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	129.1		217.1
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	372.8		293.7
Debt Securities, Available-for-sale, Total	5,299.4		5,321.6
Mortgage/Asset-backed Securities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	785.4		658.0
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	4.3		10.1
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	37.2		24.0
Debt Securities, Available-for-sale, Total	752.5		644.1
All Other Corporate Bonds
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	26,780.8		26,708.4
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	5.1	$ 2.2	2.2
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	481.8		771.8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	2,198.4		1,807.3
Debt Securities, Available-for-sale, Total	25,059.1		25,670.7
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	0.4		0.4
Debt Securities, Available-for-sale, Total	3.6		3.6
Fixed Maturity Securities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	38,554.3		38,410.6
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	5.1		2.2
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	723.4		1,189.4
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	3,319.8		2,763.9
Debt Securities, Available-for-sale, Total	$ 35,952.8		$ 36,833.9