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Amortized Cost and Fair Values of Securities by Security Type (Detail) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	$ 38,385.4	$ 38,410.6
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	2.2	2.2	$ 0.0	$ 0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	902.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	3,037.4
Debt Securities, Available-for-sale, Total	36,247.8	36,833.9
United States Government and Government Agencies and Authorities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	585.8	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	17.4	25.3
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	25.0	19.1
Debt Securities, Available-for-sale, Total	578.2	624.8
States, Municipalities, and Political Subdivisions
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	3,978.2	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	105.2	135.3
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	518.5	498.2
Debt Securities, Available-for-sale, Total	3,564.9	3,678.4
Foreign Governments
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	932.9	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	22.2	29.8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	128.3	121.2
Debt Securities, Available-for-sale, Total	826.8	890.7
Public Utilities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	5,496.5	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	163.2	217.1
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	329.5	293.7
Debt Securities, Available-for-sale, Total	5,330.2	5,321.6
Mortgage/Asset-backed Securities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	735.4	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	5.0	10.1
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	31.7	24.0
Debt Securities, Available-for-sale, Total	708.7	644.1
All Other Corporate Bonds
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	26,652.6	26,708.4
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	2.2	2.2
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	589.0	771.8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	2,004.0	1,807.3
Debt Securities, Available-for-sale, Total	25,235.4	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	38,385.4	38,410.6
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	2.2	2.2
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	902.0	1,189.4
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	3,037.4	2,763.9
Debt Securities, Available-for-sale, Total	$ 36,247.8	$ 36,833.9