Fair Value - Additional Information (Detail)	Sep. 30, 2020 USD ($) security	Dec. 31, 2019 USD ($) security	Sep. 30, 2019 USD ($) security
Fair Value Measurements [Line Items]
Investments, Fair Value	$ 45,764,300,000	$ 39,254,300,000	$ 38,573,700,000
Fair Value, Inputs, Level 1
Fair Value Measurements [Line Items]
Vendor Quoted Prices, Percent of FV Hierarchy Level	75.00%	80.00%	81.00%
Investments, Fair Value	$ 18,289,800,000	$ 18,443,000,000.0	$ 18,401,800,000
Fair Value, Inputs, Level 2
Fair Value Measurements [Line Items]
Vendor Quoted Prices, Percent of FV Hierarchy Level	99.00%	99.00%	99.00%
Dealer Quoted Prices, Percent of FV Hierarchy Level	1.00%	1.00%	1.00%
Investments, Fair Value	$ 27,418,500,000	$ 20,767,900,000	$ 20,139,500,000
Fair Value, Inputs, Level 3
Fair Value Measurements [Line Items]
Investments, Fair Value	56,000,000.0	43,400,000	32,400,000
Total fixed maturities \| Fair Value, Inputs, Level 3
Fair Value Measurements [Line Items]
Investments, Fair Value	$ 0	$ 0	$ 0
Nonredeemable preferred stocks \| Internal Pricing \| Fair Value, Inputs, Level 3
Fair Value Measurements [Line Items]
Number of securities \| security	4	3	2
Common equities \| Internal Pricing \| Fair Value, Inputs, Level 3
Fair Value Measurements [Line Items]
Number of securities \| security	2	1	1