Liability for Losses and Loss Expenses - Summary of Insurance Subsidiaries' Liability for Losses and Loss Expenses (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Liability for Claims and Claims Adjustment Expense [Abstract]
Balance at January 1	$ 606,664,590	$ 578,205,109	$ 538,258,406
Less reinsurance recoverable	(259,147,147)	(256,150,860)	(245,957,364)
Net balance at January 1	347,517,443	322,054,249	292,301,042
Incurred related to:
Current year	480,646,641	420,327,164	391,166,740
Prior years	6,621,413	2,988,739	7,200,134
Total incurred	487,268,054	423,315,903	398,366,874
Paid related to:
Current year	288,379,600	248,106,788	236,834,666
Prior years	163,005,427	149,745,921	131,779,001
Total paid	451,385,027	397,852,709	368,613,667
Net balance at December 31	383,400,470	347,517,443	322,054,249
Plus reinsurance recoverable	293,271,257	259,147,147	256,150,860
Balance at December 31	$ 676,671,727	$ 606,664,590	$ 578,205,109