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Amortized Cost and Fair Values of Securities by Security Type (Detail) - USD ($) $ in Millions	Sep. 30, 2021	Jun. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Jan. 01, 2020
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	$ 37,355.0		$ 36,546.5
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	0.0		6.8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	6,360.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	86.7
Debt Securities, Available-for-sale, Total	43,628.3		44,137.3
United States Government and Government Agencies and Authorities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	458.3		559.0
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	0.0		0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	118.6		150.8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0.0		0.0
Debt Securities, Available-for-sale, Total	576.9		709.8
States, Municipalities, and Political Subdivisions
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	4,022.6		3,609.9
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	0.0		0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	577.3		652.8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	12.5		1.5
Debt Securities, Available-for-sale, Total	4,587.4		4,261.2
Foreign Governments
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	939.9		902.9
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	0.0		0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	228.4		266.5
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	7.2		1.2
Debt Securities, Available-for-sale, Total	1,161.1		1,168.2
Public Utilities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	5,334.0		5,486.4
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	0.0		0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	1,206.2		1,481.9
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	10.8		6.0
Debt Securities, Available-for-sale, Total	6,529.4		6,962.3
Mortgage/Asset-backed Securities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	628.5		1,019.9
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	0.0		0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	61.2		88.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0.1		0.2
Debt Securities, Available-for-sale, Total	689.6		1,107.7
All Other Corporate Bonds
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	25,967.7		24,958.8
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	0.0	$ 0.0	6.8	$ 12.5	$ 29.8	$ 0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	4,168.2		5,013.5
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	56.1		46.9
Debt Securities, Available-for-sale, Total	30,079.8		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 losses for Fixed Maturity Securities	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	37,355.0		36,546.5
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	0.0		6.8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	6,360.0		7,653.5
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	86.7		55.9
Debt Securities, Available-for-sale, Total	$ 43,628.3		$ 44,137.3