Liability for Losses and Loss Expenses - Summary of Insurance Subsidiaries' Liability for Losses and Loss Expenses (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Liability for Claims and Claims Adjustment Expense [Abstract]
Balance at January 1	$ 814,665,224	$ 676,672,000
Less reinsurance recoverable	(339,267,000)	(293,271,000)
Net balance at January 1	475,398,000	383,401,000
Incurred related to:
Current year	127,117,000	129,842,000
Prior years	(4,006,000)	26,741,000
Total incurred	123,111,000	156,583,000
Paid related to:
Current year	43,232,000	54,583,000
Prior years	66,305,000	67,117,000
Total paid	109,537,000	121,700,000
Net balance at end of period	488,972,000	418,284,000
Plus reinsurance recoverable	347,548,000	309,279,000
Balance at end of period	$ 836,519,776	$ 727,563,000