Amortized Cost and Fair Values of Securities by Security Type (Detail) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021	Jan. 01, 2020
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	$ 37,825.2	$ 37,386.7
Debt Securities, Available-for-sale, Allowance for Credit Loss	0.0	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	743.6
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	3,728.0
Debt Securities, Available-for-sale, Total	34,840.8	43,336.0
United States Government and Government Agencies and Authorities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	503.8	460.1
Debt Securities, Available-for-sale, Allowance for Credit Loss	0.0	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	20.3	120.1
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	25.9	0.1
Debt Securities, Available-for-sale, Total	498.2	580.1
States, Municipalities, and Political Subdivisions
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	4,006.0	4,150.2
Debt Securities, Available-for-sale, Allowance for Credit Loss	0.0	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	87.1	584.2
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	635.9	6.9
Debt Securities, Available-for-sale, Total	3,457.2	4,727.5
Foreign Governments
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	908.1	952.0
Debt Securities, Available-for-sale, Allowance for Credit Loss	0.0	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	34.9	215.3
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	115.9	20.7
Debt Securities, Available-for-sale, Total	827.1	1,146.6
Public Utilities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	5,170.9	5,266.4
Debt Securities, Available-for-sale, Allowance for Credit Loss	0.0	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	141.0	1,159.4
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	355.0	9.8
Debt Securities, Available-for-sale, Total	4,956.9	6,416.0
Mortgage/Asset-backed Securities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	592.1	587.9
Debt Securities, Available-for-sale, Allowance for Credit Loss	0.0	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	8.2	50.4
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	27.0	0.0
Debt Securities, Available-for-sale, Total	573.3	638.3
All Other Corporate Bonds
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	26,640.3	25,966.1
Debt Securities, Available-for-sale, Allowance for Credit Loss	0.0	0.0	$ 6.8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	452.1	3,919.9
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	2,567.8	62.6
Debt Securities, Available-for-sale, Total	24,524.6	29,823.4
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	0.1
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0.5	0.0
Debt Securities, Available-for-sale, Total	3.5	4.1
Fixed Maturity Securities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	37,825.2	37,386.7
Debt Securities, Available-for-sale, Allowance for Credit Loss	0.0	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	743.6	6,049.4
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	3,728.0	100.1
Debt Securities, Available-for-sale, Total	$ 34,840.8	$ 43,336.0

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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