Amortized Cost and Fair Values of Securities by Security Type (Detail) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020	Jan. 01, 2020
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	$ 37,386.7	$ 36,546.5
Debt Securities, Available-for-sale, Allowance for Credit Loss	0.0	6.8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	6,049.4
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	100.1
Debt Securities, Available-for-sale, Total	43,336.0	44,137.3
United States Government and Government Agencies and Authorities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	460.1	559.0
Debt Securities, Available-for-sale, Allowance for Credit Loss	0.0	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	120.1	150.8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0.1	0.0
Debt Securities, Available-for-sale, Total	580.1	709.8
States, Municipalities, and Political Subdivisions
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	4,150.2	3,609.9
Debt Securities, Available-for-sale, Allowance for Credit Loss	0.0	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	584.2	652.8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	6.9	1.5
Debt Securities, Available-for-sale, Total	4,727.5	4,261.2
Foreign Governments
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	952.0	902.9
Debt Securities, Available-for-sale, Allowance for Credit Loss	0.0	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	215.3	266.5
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	20.7	1.2
Debt Securities, Available-for-sale, Total	1,146.6	1,168.2
Public Utilities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	5,266.4	5,486.4
Debt Securities, Available-for-sale, Allowance for Credit Loss	0.0	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	1,159.4	1,481.9
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	9.8	6.0
Debt Securities, Available-for-sale, Total	6,416.0	6,962.3
Mortgage/Asset-backed Securities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	587.9	1,019.9
Debt Securities, Available-for-sale, Allowance for Credit Loss	0.0	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	50.4	88.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0.0	0.2
Debt Securities, Available-for-sale, Total	638.3	1,107.7
All Other Corporate Bonds
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	25,966.1	24,958.8
Debt Securities, Available-for-sale, Allowance for Credit Loss	0.0	6.8	$ 0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	3,919.9	5,013.5
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	62.6	46.9
Debt Securities, Available-for-sale, Total	29,823.4	29,918.6
Redeemable Preferred Stocks
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	4.0	9.6
Debt Securities, Available-for-sale, Allowance for Credit Loss	0.0	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	0.1	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0.0	0.1
Debt Securities, Available-for-sale, Total	4.1	9.5
Fixed Maturity Securities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	37,386.7	36,546.5
Debt Securities, Available-for-sale, Allowance for Credit Loss	0.0	6.8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	6,049.4	7,653.5
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	100.1	55.9
Debt Securities, Available-for-sale, Total	$ 43,336.0	$ 44,137.3

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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