Amortized Cost and Fair Values of Securities by Security Type (Detail) - USD ($) $ in Millions	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	$ 41,239.4		$ 41,079.3
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	12.5		0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	8,177.1
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	146.3
Debt Securities, Available-for-sale, Total	49,270.2		47,443.7
United States Government and Government Agencies and Authorities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	1,032.0		1,246.1
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	217.9		156.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0.0		0.4
Debt Securities, Available-for-sale, Total	1,249.9		1,401.7
States, Municipalities, and Political Subdivisions
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	3,321.6		2,863.1
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	612.0		507.6
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	3.6		7.3
Debt Securities, Available-for-sale, Total	3,930.0		3,363.4
Foreign Governments
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	860.9		843.5
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	255.6		175.2
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	1.4		1.0
Debt Securities, Available-for-sale, Total	1,115.1		1,017.7
Public Utilities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	6,359.9		6,436.7
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	1,662.7		1,303.7
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	6.6		8.2
Debt Securities, Available-for-sale, Total	8,016.0		7,732.2
Mortgage/Asset-backed Securities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	1,317.1		1,377.8
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	115.2		101.3
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0.2		0.4
Debt Securities, Available-for-sale, Total	1,432.1		1,478.7
All Other Corporate Bonds
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	28,338.3		28,273.1
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	12.5	$ 29.8	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	5,313.7		4,211.2
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	134.4		73.9
Debt Securities, Available-for-sale, Total	33,517.6		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 losses for Fixed Maturity Securities	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	41,239.4		41,079.3
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	12.5
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	8,177.1		6,455.6
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	146.3		91.2
Debt Securities, Available-for-sale, Total	$ 49,270.2		$ 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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