Liability for Losses and Loss Expenses - Summary of Insurance Subsidiaries' Liability for Losses and Loss Expenses (Detail) - USD ($)	6 Months Ended
	Jun. 30, 2017	Jun. 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	234,045,000	195,036,000
Prior years	8,394,000	3,736,000
Total incurred	242,439,000	198,772,000
Paid related to:
Current year	120,392,000	98,170,000
Prior years	99,224,000	96,392,000
Total paid	219,616,000	194,562,000
Net balance at end of period	370,341,000	326,264,000
Plus reinsurance recoverable	272,905,000	250,901,000
Balance at end of period	$ 643,246,130	$ 577,165,000