Fair Value - Additional Information (Detail)	Mar. 31, 2021 USD ($) security	Dec. 31, 2020 USD ($) security	Mar. 31, 2020 USD ($) security
Fair Value Measurements [Line Items]
Investments, Fair Value	$ 47,400,600,000	$ 47,530,300,000	$ 40,342,300,000
Fair Value, Inputs, Level 1
Fair Value Measurements [Line Items]
Vendor Quoted Prices, Percent of FV Hierarchy Level	78.00%	76.00%	82.00%
Investments, Fair Value	$ 22,980,900,000	$ 21,623,300,000	$ 16,654,700,000
Fair Value, Inputs, Level 2
Fair Value Measurements [Line Items]
Vendor Quoted Prices, Percent of FV Hierarchy Level	99.00%	99.00%	98.00%
Dealer Quoted Prices, Percent of FV Hierarchy Level	1.00%	1.00%	2.00%
Investments, Fair Value	$ 24,364,900,000	$ 25,852,200,000	$ 23,659,000,000.0
Fair Value, Inputs, Level 3
Fair Value Measurements [Line Items]
Investments, Fair Value	54,800,000	54,800,000	28,600,000
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	4	4	3
Common equities | Fair Value, Inputs, Level 3
Fair Value Measurements [Line Items]
Number of securities | security	2	2	1