Investments - Additional Information (Detail)	3 Months Ended	9 Months Ended	12 Months Ended
Sep. 30, 2023 USD ($) security	Sep. 30, 2023 USD ($) security	Sep. 30, 2022 USD ($)	Dec. 31, 2022 USD ($)
Schedule of Debt and Equity Securities [Line Items]
Principal amount of bonds and certificates of deposit	$ 612,400,000	$ 612,400,000
Securities exceeding 10% of Shareholders' Equity	0	0	$ 0	$ 0
Non-income producing securities	0	0
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, number of positions downgraded \| security	14	14
Debt securities, available-for-sale, unrealized loss position, fair value, downgraded in period	$ 144,200,000	$ 144,200,000
Debt securities, available-for-sale, unrealized loss position, accumulated loss, downgraded in period	7,100,000	7,100,000
Debt securities, available-for-sale, allowance for credit loss, not previously recorded		0	0
Debt securities, available-for-sale, allowance for credit loss	$ 0	0	0	0
Debt securities, available-for-sale, allowance for credit loss, write-off		$ 0	$ 0	$ 0
Recovery of impairment, other assets, number of positions \| security		3
Percent change in investment income	59.00%	53.00%
Percent change in book yield	32.00%	39.00%

Amount of accumulated unrealized loss on debt securities with credit rating downgrades during the period. These securities are measured at fair value with changes in fair value recognized in other comprehensive income (available-for-sale), in an unrealized loss position, without a credit loss allowance.

No definition available.