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Amortized Cost and Fair Values of Securities by Security Type (Detail) - USD ($) $ in Millions	Sep. 30, 2022	Jun. 30, 2022	Dec. 31, 2021	Sep. 30, 2021	Jun. 30, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	$ 37,721.4		$ 37,386.7
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	4.6		0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	638.5
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	4,207.1
Debt Securities, Available-for-sale, Total	34,148.2		43,336.0
United States Government and Government Agencies and Authorities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	466.2		460.1
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	23.1		120.1
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	23.0		0.1
Debt Securities, Available-for-sale, Total	466.3		580.1
States, Municipalities, and Political Subdivisions
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	4,286.9		4,150.2
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	80.4		584.2
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	731.8		6.9
Debt Securities, Available-for-sale, Total	3,635.5		4,727.5
Foreign Governments
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	872.6		952.0
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	36.3		215.3
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	99.8		20.7
Debt Securities, Available-for-sale, Total	809.1		1,146.6
Public Utilities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	5,175.4		5,266.4
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	4.6	$ 4.1	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	126.7		1,159.4
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	386.6		9.8
Debt Securities, Available-for-sale, Total	4,910.9		6,416.0
Mortgage/Asset-backed Securities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	555.0		587.9
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	7.4		50.4
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	32.1		0.0
Debt Securities, Available-for-sale, Total	530.3		638.3
All Other Corporate Bonds
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	26,361.3		25,966.1
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	0.0		0.0	$ 0.0	$ 0.0	$ 6.8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	364.6		3,919.9
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	2,933.3		62.6
Debt Securities, Available-for-sale, Total	23,792.6		29,823.4
Redeemable Preferred Stocks
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	4.0		4.0
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	0.0		0.1
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0.5		0.0
Debt Securities, Available-for-sale, Total	3.5		4.1
Fixed Maturity Securities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	37,721.4		37,386.7
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	4.6		0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	638.5		6,049.4
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	4,207.1		100.1
Debt Securities, Available-for-sale, Total	$ 34,148.2		$ 43,336.0