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Amortized Cost and Fair Values of Securities by Security Type (Detail) - USD ($) $ in Millions	Sep. 30, 2024	Jun. 30, 2024	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	$ 38,822.6		$ 38,410.6
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	2.9		2.2	$ 0.0	$ 0.0	$ 0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	1,338.2
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	2,374.4
Debt Securities, Available-for-sale, Total	37,783.5		36,833.9
United States Government and Government Agencies and Authorities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	542.5		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	28.8		25.3
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	13.9		19.1
Debt Securities, Available-for-sale, Total	557.4		624.8
States, Municipalities, and Political Subdivisions
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	3,843.4		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	118.7		135.3
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	435.6		498.2
Debt Securities, Available-for-sale, Total	3,526.5		3,678.4
Foreign Governments
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	964.8		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	22.4		29.8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	137.1		121.2
Debt Securities, Available-for-sale, Total	850.1		890.7
Public Utilities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	5,561.8		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	272.3		217.1
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	248.9		293.7
Debt Securities, Available-for-sale, Total	5,585.2		5,321.6
Mortgage/Asset-backed Securities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	853.8		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	13.2		10.1
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	16.4		24.0
Debt Securities, Available-for-sale, Total	850.6		644.1
All Other Corporate Bonds
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	27,048.3		26,708.4
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	2.9	$ 5.1	2.2
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	882.8		771.8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	1,522.2		1,807.3
Debt Securities, Available-for-sale, Total	26,406.0		25,670.7
Redeemable Preferred Stocks
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	8.0		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	0.3		0.4
Debt Securities, Available-for-sale, Total	7.7		3.6
Fixed Maturity Securities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	38,822.6		38,410.6
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	2.9		2.2
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	1,338.2		1,189.4
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	$ 2,374.4		$ 2,763.9