Amortized Cost and Fair Values of Securities by Security Type (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	$ 36,546.5	$ 41,079.3
Debt Securities, Available-for-sale, Allowance for Credit Loss	6.8	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	7,653.5
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	55.9
Debt Securities, Available-for-sale, Total	44,137.3	47,443.7
United States Government and Government Agencies and Authorities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	559.0	1,246.1
Debt Securities, Available-for-sale, Allowance for Credit Loss	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	150.8	156.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0.0	0.4
Debt Securities, Available-for-sale, Total	709.8	1,401.7
States, Municipalities, and Political Subdivisions
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	3,609.9	2,863.1
Debt Securities, Available-for-sale, Allowance for Credit Loss	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	652.8	507.6
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	1.5	7.3
Debt Securities, Available-for-sale, Total	4,261.2	3,363.4
Foreign Governments
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	902.9	843.5
Debt Securities, Available-for-sale, Allowance for Credit Loss	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	266.5	175.2
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	1.2	1.0
Debt Securities, Available-for-sale, Total	1,168.2	1,017.7
Public Utilities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	5,486.4	6,436.7
Debt Securities, Available-for-sale, Allowance for Credit Loss	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	1,481.9	1,303.7
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	6.0	8.2
Debt Securities, Available-for-sale, Total	6,962.3	7,732.2
Mortgage/Asset-backed Securities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	1,019.9	1,377.8
Debt Securities, Available-for-sale, Allowance for Credit Loss	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	88.0	101.3
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0.2	0.4
Debt Securities, Available-for-sale, Total	1,107.7	1,478.7
All Other Corporate Bonds
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	24,958.8	28,273.1
Debt Securities, Available-for-sale, Allowance for Credit Loss	6.8	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	5,013.5	4,211.2
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	46.9	73.9
Debt Securities, Available-for-sale, Total	29,918.6	32,410.4
Redeemable Preferred Stocks
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	9.6	39.0
Debt Securities, Available-for-sale, Allowance for Credit Loss	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	0.0	0.6
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0.1	0.0
Debt Securities, Available-for-sale, Total	9.5	39.6
Fixed Maturity Securities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	36,546.5	41,079.3
Debt Securities, Available-for-sale, Allowance for Credit Loss	6.8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	7,653.5	6,455.6
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	55.9	91.2
Debt Securities, Available-for-sale, Total	$ 44,137.3	$ 47,443.7

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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