Liability for Losses and Loss Expenses - Summary of Insurance Subsidiaries' Liability for Losses and Loss Expenses (Detail) - USD ($)	6 Months Ended
	Jun. 30, 2018	Jun. 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	266,093,000	234,045,000
Prior years	26,244,000	8,394,000
Total incurred	292,337,000	242,439,000
Paid related to:
Current year	127,392,000	120,392,000
Prior years	108,521,000	99,224,000
Total paid	235,913,000	219,616,000
Net balance at end of period	439,825,000	370,341,000
Plus reinsurance recoverable	307,805,000	272,905,000
Balance at end of period	$ 747,630,145	$ 643,246,000