Amortized Cost and Fair Values of Securities by Security Type (Detail) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale
Debt Securities, Available-for-sale, Amortized Cost, Total	$ 41,030.3	$ 41,079.3
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	5,328.4
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	1,067.2
Debt Securities, Available-for-sale, Total	45,291.5	47,443.7
United States Government and Government Agencies and Authorities
Debt Securities, Available-for-sale
Debt Securities, Available-for-sale, Amortized Cost, Total	1,155.6	1,246.1
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	227.1	156.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0.0	0.4
Debt Securities, Available-for-sale, Total	1,382.7	1,401.7
States, Municipalities, and Political Subdivisions
Debt Securities, Available-for-sale
Debt Securities, Available-for-sale, Amortized Cost, Total	3,129.0	2,863.1
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	528.2	507.6
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	9.8	7.3
Debt Securities, Available-for-sale, Total	3,647.4	3,363.4
Foreign Governments
Debt Securities, Available-for-sale
Debt Securities, Available-for-sale, Amortized Cost, Total	786.3	843.5
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	174.2	175.2
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	2.2	1.0
Debt Securities, Available-for-sale, Total	958.3	1,017.7
Public Utilities
Debt Securities, Available-for-sale
Debt Securities, Available-for-sale, Amortized Cost, Total	6,361.7	6,436.7
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	1,231.6	1,303.7
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	42.4	8.2
Debt Securities, Available-for-sale, Total	7,550.9	7,732.2
Mortgage/Asset-backed Securities
Debt Securities, Available-for-sale
Debt Securities, Available-for-sale, Amortized Cost, Total	1,316.7	1,377.8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	129.9	101.3
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0.9	0.4
Debt Securities, Available-for-sale, Total	1,445.7	1,478.7
All Other Corporate Bonds
Debt Securities, Available-for-sale
Debt Securities, Available-for-sale, Amortized Cost, Total	28,238.0	28,273.1
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	3,037.4	4,211.2
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	1,011.3	73.9
Debt Securities, Available-for-sale, Total	30,264.1	32,410.4
Redeemable Preferred Stocks
Debt Securities, Available-for-sale
Debt Securities, Available-for-sale, Amortized Cost, Total	43.0	39.0
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	0.6	0.0
Debt Securities, Available-for-sale, Total	42.4	39.6
Fixed Maturity Securities
Debt Securities, Available-for-sale
Debt Securities, Available-for-sale, Amortized Cost, Total	41,030.3	41,079.3
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	5,328.4	6,455.6
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	1,067.2	91.2
Debt Securities, Available-for-sale, Total	$ 45,291.5	$ 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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