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Amortized Cost and Fair Values of Securities by Security Type (Detail) - USD ($) $ in Millions	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Jun. 30, 2020	Mar. 31, 2020	Jan. 01, 2020
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	$ 36,910.7		$ 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	6,663.2
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	60.1
Debt Securities, Available-for-sale, Total	43,513.8		44,137.3
United States Government and Government Agencies and Authorities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	469.2		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	122.9		150.8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0.0		0.0
Debt Securities, Available-for-sale, Total	592.1		709.8
States, Municipalities, and Political Subdivisions
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	3,865.8		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	607.2		652.8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	4.7		1.5
Debt Securities, Available-for-sale, Total	4,468.3		4,261.2
Foreign Governments
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	913.1		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	232.9		266.5
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	2.6		1.2
Debt Securities, Available-for-sale, Total	1,143.4		1,168.2
Public Utilities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	5,382.2		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	1,246.2		1,481.9
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	11.0		6.0
Debt Securities, Available-for-sale, Total	6,617.4		6,962.3
Mortgage/Asset-backed Securities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	715.6		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	65.7		88.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0.1		0.2
Debt Securities, Available-for-sale, Total	781.2		1,107.7
All Other Corporate Bonds
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	25,560.8		24,958.8
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	0.0	$ 7.3	6.8	$ 29.8	$ 48.0	$ 0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	4,388.1		5,013.5
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	41.7		46.9
Debt Securities, Available-for-sale, Total	29,907.2		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	0.2		0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0.0		0.1
Debt Securities, Available-for-sale, Total	4.2		9.5
Fixed Maturity Securities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	36,910.7		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	6,663.2		7,653.5
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	60.1		55.9
Debt Securities, Available-for-sale, Total	$ 43,513.8		$ 44,137.3