Liability for Losses and Loss Expenses - Summary of Insurance Subsidiaries' Liability for Losses and Loss Expenses (Detail) - USD ($)	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Liability for Claims and Claims Adjustment Expense [Abstract]
Balance at January 1	$ 606,664,590	$ 578,205,000
Less reinsurance recoverable	(259,147,000)	(256,151,000)
Net balance at January 1	347,518,000	322,054,000
Incurred related to:
Current year	351,812,000	307,826,000
Prior years	5,014,000	2,121,000
Total incurred	356,826,000	309,947,000
Paid related to:
Current year	201,849,000	169,798,000
Prior years	133,587,000	122,162,000
Total paid	335,436,000	291,960,000
Net balance at end of period	368,908,000	340,041,000
Plus reinsurance recoverable	275,442,000	254,227,000
Balance at end of period	$ 644,350,129	$ 594,268,000