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Amortized Cost and Fair Values of Securities by Security Type (Detail) - USD ($) $ in Millions	Mar. 31, 2023	Dec. 31, 2022	Mar. 31, 2022	Dec. 31, 2021
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	$ 38,101.8	$ 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	1,085.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	3,026.6
Debt Securities, Available-for-sale, Total	36,160.2	34,840.8
United States Government and Government Agencies and Authorities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	529.9	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	28.8	20.3
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	17.4	25.9
Debt Securities, Available-for-sale, Total	541.3	498.2
States, Municipalities, and Political Subdivisions
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	4,012.1	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	132.1	87.1
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	503.0	635.9
Debt Securities, Available-for-sale, Total	3,641.2	3,457.2
Foreign Governments
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	935.1	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	46.4	34.9
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	105.5	115.9
Debt Securities, Available-for-sale, Total	876.0	827.1
Public Utilities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	5,301.5	5,170.9
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	0.0	0.0	$ 4.1	$ 0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	227.9	141.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	285.0	355.0
Debt Securities, Available-for-sale, Total	5,244.4	4,956.9
Mortgage/Asset-backed Securities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	586.7	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	9.9	8.2
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	20.3	27.0
Debt Securities, Available-for-sale, Total	576.3	573.3
All Other Corporate Bonds
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	26,732.5	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	639.9	452.1
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	2,095.0	2,567.8
Debt Securities, Available-for-sale, Total	25,277.4	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	0.4	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	38,101.8	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	1,085.0	743.6
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	3,026.6	3,728.0
Debt Securities, Available-for-sale, Total	$ 36,160.2	$ 34,840.8