Amortized Cost and Fair Values of Securities by Security Type (Detail) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	$ 38,410.6	$ 37,825.2
Debt Securities, Available-for-sale, Allowance for Credit Loss	2.2	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	1,189.4
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	2,763.9
Debt Securities, Available-for-sale, Total	36,833.9	34,840.8
United States Government and Government Agencies and Authorities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	618.6	503.8
Debt Securities, Available-for-sale, Allowance for Credit Loss	0.0	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	25.3	20.3
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	19.1	25.9
Debt Securities, Available-for-sale, Total	624.8	498.2
States, Municipalities, and Political Subdivisions
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	4,041.3	4,006.0
Debt Securities, Available-for-sale, Allowance for Credit Loss	0.0	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	135.3	87.1
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	498.2	635.9
Debt Securities, Available-for-sale, Total	3,678.4	3,457.2
Foreign Governments
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	982.1	908.1
Debt Securities, Available-for-sale, Allowance for Credit Loss	0.0	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	29.8	34.9
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	121.2	115.9
Debt Securities, Available-for-sale, Total	890.7	827.1
Public Utilities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	5,398.2	5,170.9
Debt Securities, Available-for-sale, Allowance for Credit Loss	0.0	0.0	$ 0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	217.1	141.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	293.7	355.0
Debt Securities, Available-for-sale, Total	5,321.6	4,956.9
Mortgage/Asset-backed Securities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	658.0	592.1
Debt Securities, Available-for-sale, Allowance for Credit Loss	0.0	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	10.1	8.2
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	24.0	27.0
Debt Securities, Available-for-sale, Total	644.1	573.3
All Other Corporate Bonds
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	26,708.4	26,640.3
Debt Securities, Available-for-sale, Allowance for Credit Loss	2.2	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	771.8	452.1
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	1,807.3	2,567.8
Debt Securities, Available-for-sale, Total	25,670.7	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 Loss	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	0.5
Debt Securities, Available-for-sale, Total	3.6	3.5
Fixed Maturity Securities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	38,410.6	37,825.2
Debt Securities, Available-for-sale, Allowance for Credit Loss	2.2	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	1,189.4	743.6
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	2,763.9	3,728.0
Debt Securities, Available-for-sale, Total	$ 36,833.9	$ 34,840.8

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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