Fair Value - Additional Information (Detail)	Sep. 30, 2021 USD ($) security	Dec. 31, 2020 USD ($) security	Sep. 30, 2020 USD ($) security
Fair Value Measurements [Line Items]
Investments, Fair Value	$ 52,287,400,000	$ 47,530,300,000	$ 45,764,300,000
Fair Value, Inputs, Level 1
Fair Value Measurements [Line Items]
Vendor Quoted Prices, Percent of FV Hierarchy Level	82.00%	76.00%	75.00%
Investments, Fair Value	$ 26,834,100,000	$ 21,623,300,000	$ 18,289,800,000
Fair Value, Inputs, Level 2
Fair Value Measurements [Line Items]
Vendor Quoted Prices, Percent of FV Hierarchy Level	97.00%	99.00%	99.00%
Dealer Quoted Prices, Percent of FV Hierarchy Level	3.00%	1.00%	1.00%
Investments, Fair Value	$ 25,275,400,000	$ 25,852,200,000	$ 27,418,500,000
Fair Value, Inputs, Level 3
Fair Value Measurements [Line Items]
Investments, Fair Value	177,900,000	54,800,000	56,000,000.0
Total fixed maturities | Fair Value, Inputs, Level 3
Fair Value Measurements [Line Items]
Investments, Fair Value	$ 0	$ 0	$ 0
Nonredeemable preferred stocks | Fair Value, Inputs, Level 3
Fair Value Measurements [Line Items]
Number of securities | security	7	3	3
Common equities | Fair Value, Inputs, Level 3 | Other risk investments
Fair Value Measurements [Line Items]
Number of securities | security	6	2	2
Common equities | Fair Value, Inputs, Level 3 | Common Stocks, excluding Other Risk
Fair Value Measurements [Line Items]
Number of securities | security	1	1	1