Liability for Losses and Loss Expenses - Summary of Insurance Subsidiaries' Liability for Losses and Loss Expenses (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2017	Mar. 31, 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	111,872,000	95,584,000
Prior years	2,561,000	(6,346)
Total incurred	114,433,000	95,578,000
Paid related to:
Current year	45,401,000	38,695,000
Prior years	60,759,000	54,192,000
Total paid	106,160,000	92,887,000
Net balance at end of period	355,791,000	324,745,000
Plus reinsurance recoverable	265,058,000	249,800,000
Balance at end of period	$ 620,849,300	$ 574,545,000