Investments - Additional Information (Detail)	3 Months Ended	9 Months Ended
	Sep. 30, 2021 USD ($) security	Sep. 30, 2021 USD ($) security	Dec. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)
Schedule of Debt and Equity Securities [Line Items]
Principal amount of bonds and certificates of deposit	$ 407,800,000	$ 407,800,000
Securities Exceeding 10% of Shareholders' Equity	0	0	$ 0	$ 0
Non-Income Producing Securities	0	0
Total available-for-sale securities	46,134,400,000	46,134,400,000	42,029,400,000	40,981,300,000
Open reverse repurchase commitments	0	0	0	0
Securities Sold under Agreements to Repurchase	$ 0	$ 0	0	0
Debt Securities, Available-for-sale, Unrealized Loss Position, Positions Downgraded | security	2	2
Combined fair value of securities	$ 10,500,000	$ 10,500,000
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Downgrades in Period	400,000	400,000
Debt Securities, Available-for-sale, Allowance for Credit Loss, Not Previously Recorded		0
Debt Securities, Available-for-sale, Allowance for Credit Loss	$ 0	0	$ 0	$ 0
Debt Securities, Available-for-sale, Allowance for Credit Loss, Writeoff		$ 0
Percent Change in Investment Income	9.00%	11.00%
Percent Change in Book Yield	21.00%	23.00%
Foreign Government Debt
Schedule of Debt and Equity Securities [Line Items]
Total available-for-sale securities	$ 12,300,000	$ 12,300,000