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Amortized Cost and Fair Values of Securities by Security Type (Detail) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021	Mar. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	$ 37,669.7	$ 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	3,205.3
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	924.9
Debt Securities, Available-for-sale, Total	39,946.0	43,336.0
United States Government and Government Agencies and Authorities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	461.8	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	77.0	120.1
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	1.5	0.1
Debt Securities, Available-for-sale, Total	537.3	580.1
States, Municipalities, and Political Subdivisions
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	4,223.4	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	307.3	584.2
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	223.4	6.9
Debt Securities, Available-for-sale, Total	4,307.3	4,727.5
Foreign Governments
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	933.7	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	179.2	215.3
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	48.2	20.7
Debt Securities, Available-for-sale, Total	1,064.7	1,146.6
Public Utilities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	5,235.7	5,266.4
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	687.4	1,159.4
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	70.4	9.8
Debt Securities, Available-for-sale, Total	5,848.6	6,416.0
Mortgage/Asset-backed Securities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	536.4	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	23.4	50.4
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0.7	0.0
Debt Securities, Available-for-sale, Total	559.1	638.3
All Other Corporate Bonds
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	26,274.7	25,966.1
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	0.0	0.0	$ 7.3	$ 6.8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	1,931.0	3,919.9
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	580.5	62.6
Debt Securities, Available-for-sale, Total	27,625.2	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	0.2	0.0
Debt Securities, Available-for-sale, Total	3.8	4.1
Fixed Maturity Securities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	37,669.7	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	3,205.3	6,049.4
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	924.9	100.1
Debt Securities, Available-for-sale, Total	$ 39,946.0	$ 43,336.0