Liabilities for Losses and Loss Expenses - Summary of Insurance Subsidiaries' Liabilities for Losses and Loss Expenses (Detail) - USD ($)	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Liability for Claims and Claims Adjustment Expense [Abstract]
Balance at January 1	$ 962,007,437	$ 869,674,000
Less reinsurance recoverable	(404,818,000)	(362,768,000)
Net balance at January 1	557,189,000	506,906,000
Incurred related to:
Current year	407,193,000	353,798,000
Prior years	(25,874,000)	(10,321,000)
Total incurred	381,319,000	343,477,000
Paid related to:
Current year	183,165,000	169,610,000
Prior years	149,910,000	139,024,000
Total paid	333,075,000	308,634,000
Net balance at end of period	605,433,000	541,749,000
Plus reinsurance recoverable	444,730,000	400,181,000
Balance at end of period	$ 1,050,163,161	$ 941,930,000