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Fair Value - Additional Information (Detail)	Sep. 30, 2023 USD ($) security	Dec. 31, 2022 USD ($)	Sep. 30, 2022 USD ($)
Fair Value Measurements [Line Items]
Investments, fair value	$ 61,870,500,000	$ 53,548,300,000	$ 52,330,400,000
Fair Value, Inputs, Level 1
Fair Value Measurements [Line Items]
Vendor quoted prices, percent of FV hierarchy level	93.00%		90.00%
Investments, fair value	$ 38,630,600,000	$ 30,799,700,000	$ 29,292,200,000
Fair Value, Inputs, Level 2
Fair Value Measurements [Line Items]
Vendor quoted prices, percent of FV hierarchy level	99.00%	98.00%	98.00%
Dealer quoted prices, percent of FV hierarchy level	1.00%	2.00%	2.00%
Investments, fair value	$ 23,135,300,000	$ 22,626,700,000	$ 22,937,600,000
Fair Value, Inputs, Level 3
Fair Value Measurements [Line Items]
Investments, fair value	$ 104,600,000	121,900,000	100,600,000
Fair Value, Inputs, Level 3 | Nonredeemable preferred stocks
Fair Value Measurements [Line Items]
Security type reclassification, level 3 securities | security	1
Available-for-sale Securities
Fair Value Measurements [Line Items]
Investments, fair value	$ 58,387,100,000	49,513,600,000	48,410,700,000
Available-for-sale Securities | Total fixed maturities
Fair Value Measurements [Line Items]
Investments, fair value	56,591,900,000	46,651,900,000	44,173,100,000
Available-for-sale Securities | Fair Value, Inputs, Level 1
Fair Value Measurements [Line Items]
Investments, fair value	36,057,000,000	27,977,300,000	26,634,000,000
Available-for-sale Securities | Fair Value, Inputs, Level 1 | Total fixed maturities
Fair Value Measurements [Line Items]
Investments, fair value	34,293,800,000	25,176,600,000	22,414,400,000
Available-for-sale Securities | Fair Value, Inputs, Level 2
Fair Value Measurements [Line Items]
Investments, fair value	22,330,100,000	21,536,300,000	21,776,700,000
Available-for-sale Securities | Fair Value, Inputs, Level 2 | Total fixed maturities
Fair Value Measurements [Line Items]
Investments, fair value	22,298,100,000	21,475,300,000	21,758,700,000
Available-for-sale Securities | Fair Value, Inputs, Level 3
Fair Value Measurements [Line Items]
Investments, fair value	0	0	0
Available-for-sale Securities | Fair Value, Inputs, Level 3 | Total fixed maturities
Fair Value Measurements [Line Items]
Investments, fair value	$ 0	$ 0	$ 0

X

- Definition

The percentage of securities with prices quoted by dealers, calculated based on the total Fair Value in the hierarchy level (Level 1, Level 2, or Level 3).

+ References

No definition available.

+ Details

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- Definition

Reclassification of existing position between security types due to conversion option or other corporate action. Value represents number of positions reclassified during the period.

+ References

No definition available.

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Fair Value Measurements [Line Items]

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- Definition

The percentage of securities with prices quoted by vendors, calculated based on the total Fair Value in the hierarchy level (Level 1, Level 2, or Level 3).

+ References

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- Definition

Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482106/820-10-50-2

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