Liability for Losses and Loss Expenses - Summary of Insurance Subsidiaries' Liability for Losses and Loss Expenses (Detail) - USD ($)	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Liability for Claims and Claims Adjustment Expense [Abstract]
Balance at January 1	$ 676,671,727	$ 606,665,000
Less reinsurance recoverable	(293,271,000)	(259,147,000)
Net balance at January 1	383,401,000	347,518,000
Incurred related to:
Current year	404,150,000	351,812,000
Prior years	28,913,000	5,014,000
Total incurred	433,063,000	356,826,000
Paid related to:
Current year	214,825,000	201,849,000
Prior years	140,806,000	133,587,000
Total paid	355,631,000	335,436,000
Net balance at end of period	460,833,000	368,908,000
Plus reinsurance recoverable	319,147,000	275,442,000
Balance at end of period	$ 779,979,677	$ 644,350,000