Liability for Losses and Loss Expenses - Summary of Insurance Subsidiaries' Liability for Losses and Loss Expenses (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2020	Mar. 31, 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	121,551,000	127,117,000
Prior years	(4,304,000)	(4,006,000)
Total incurred	117,247,000	123,111,000
Paid related to:
Current year	39,689,000	43,232,000
Prior years	68,050,000	66,305,000
Total paid	107,739,000	109,537,000
Net balance at end of period	516,414,000	488,972,000
Plus reinsurance recoverable	371,798,000	347,548,000
Balance at end of period	$ 888,212,299	$ 836,520,000