Liability for Losses and Loss Expenses - Summary of Insurance Subsidiaries' Liability for Losses and Loss Expenses (Detail) - USD ($)	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Liability for Claims and Claims Adjustment Expense [Abstract]
Balance at January 1	$ 869,673,849	$ 814,665,000
Less reinsurance recoverable	(362,768,000)	(339,267,000)
Net balance at January 1	506,906,000	475,398,000
Incurred related to:
Current year	353,798,000	393,301,000
Prior years	(10,321,000)	(7,940,000)
Total incurred	343,477,000	385,361,000
Paid related to:
Current year	169,610,000	197,234,000
Prior years	139,024,000	157,691,000
Total paid	308,634,000	354,925,000
Net balance at end of period	541,749,000	505,834,000
Plus reinsurance recoverable	400,181,000	358,700,000
Balance at end of period	$ 941,930,342	$ 864,534,000