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Amortized Cost and Fair Values of Securities by Security Type (Detail) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Jan. 01, 2020
Debt Securities, Available-for-sale
Debt Securities, Available-for-sale, Amortized Cost, Total	$ 36,565.9	$ 36,546.5
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	7.3	6.8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	5,695.1
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	177.7
Debt Securities, Available-for-sale, Total	42,076.0	44,137.3
United States Government and Government Agencies and Authorities
Debt Securities, Available-for-sale
Debt Securities, Available-for-sale, Amortized Cost, Total	464.6	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	112.1	150.8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0.1	0.0
Debt Securities, Available-for-sale, Total	576.6	709.8
States, Municipalities, and Political Subdivisions
Debt Securities, Available-for-sale
Debt Securities, Available-for-sale, Amortized Cost, Total	3,735.9	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	506.7	652.8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	38.7	1.5
Debt Securities, Available-for-sale, Total	4,203.9	4,261.2
Foreign Governments
Debt Securities, Available-for-sale
Debt Securities, Available-for-sale, Amortized Cost, Total	899.0	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	224.3	266.5
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	2.7	1.2
Debt Securities, Available-for-sale, Total	1,120.6	1,168.2
Public Utilities
Debt Securities, Available-for-sale
Debt Securities, Available-for-sale, Amortized Cost, Total	5,390.6	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	1,080.0	1,481.9
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	26.1	6.0
Debt Securities, Available-for-sale, Total	6,444.5	6,962.3
Mortgage/Asset-backed Securities
Debt Securities, Available-for-sale
Debt Securities, Available-for-sale, Amortized Cost, Total	851.6	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	73.6	88.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0.1	0.2
Debt Securities, Available-for-sale, Total	925.1	1,107.7
All Other Corporate Bonds
Debt Securities, Available-for-sale
Debt Securities, Available-for-sale, Amortized Cost, Total	25,220.2	24,958.8
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	7.3	6.8	$ 48.0	$ 0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	3,698.3	5,013.5
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	110.0	46.9
Debt Securities, Available-for-sale, Total	28,801.2	29,918.6
Redeemable Preferred Stocks
Debt Securities, Available-for-sale
Debt Securities, Available-for-sale, Amortized Cost, Total	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
Debt Securities, Available-for-sale, Amortized Cost, Total	36,565.9	36,546.5
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	7.3	6.8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	5,695.1	7,653.5
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	177.7	55.9
Debt Securities, Available-for-sale, Total	$ 42,076.0	$ 44,137.3