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Amortized Cost and Fair Values of Securities by Security Type (Detail) - USD ($) $ in Millions	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	$ 37,702.5		$ 37,386.7
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	4.1		0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	1,479.6
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	2,601.6
Debt Securities, Available-for-sale, Total	36,576.4		43,336.0
United States Government and Government Agencies and Authorities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	464.4		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	49.6		120.1
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	9.3		0.1
Debt Securities, Available-for-sale, Total	504.7		580.1
States, Municipalities, and Political Subdivisions
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	4,254.1		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	176.4		584.2
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	500.1		6.9
Debt Securities, Available-for-sale, Total	3,930.4		4,727.5
Foreign Governments
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	909.2		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	89.5		215.3
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	97.3		20.7
Debt Securities, Available-for-sale, Total	901.4		1,146.6
Public Utilities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	5,256.2		5,266.4
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	4.1	$ 4.1	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	345.2		1,159.4
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	208.9		9.8
Debt Securities, Available-for-sale, Total	5,388.4		6,416.0
Mortgage/Asset-backed Securities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	553.9		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	13.8		50.4
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	7.8		0.0
Debt Securities, Available-for-sale, Total	559.9		638.3
All Other Corporate Bonds
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	26,260.7		25,966.1
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	0.0		0.0	$ 0.0	$ 7.3	$ 6.8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	805.1		3,919.9
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	1,777.9		62.6
Debt Securities, Available-for-sale, Total	25,287.9		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	0.3		0.0
Debt Securities, Available-for-sale, Total	3.7		4.1
Fixed Maturity Securities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	37,702.5		37,386.7
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	4.1		0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	1,479.6		6,049.4
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	2,601.6		100.1
Debt Securities, Available-for-sale, Total	$ 36,576.4		$ 43,336.0