Fair Value - Additional Information (Detail)	Sep. 30, 2019 USD ($) security	Dec. 31, 2018 USD ($) security	Sep. 30, 2018 USD ($) security
Fair Value Measurements [Line Items]
Investments, Fair Value	$ 38,573,700,000	$ 33,567,400,000	$ 32,350,600,000
Fair Value, Inputs, Level 1
Fair Value Measurements [Line Items]
Vendor Quoted Prices, Percent of FV Hierarchy Level	81.00%	79.00%	76.00%
Investments, Fair Value	$ 18,401,800,000	$ 14,345,800,000	$ 15,469,300,000
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	$ 20,139,500,000	$ 19,191,200,000	$ 16,876,000,000.0
Fair Value, Inputs, Level 3
Fair Value Measurements [Line Items]
Investments, Fair Value	32,400,000	30,400,000	5,300,000
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]
Investments, Fair Value	$ 32,100,000	$ 30,100,000	$ 5,000,000.0
Number of securities | security	2	2	1
Common equities | Internal Pricing | Fair Value, Inputs, Level 3
Fair Value Measurements [Line Items]
Investments, Fair Value	$ 300,000	$ 300,000	$ 300,000
Number of securities | security	1	1	1