Reinsurance - Amounts Represent Effect of Reinsurance on Premiums Earned (Detail) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reinsurance Disclosures [Abstract]
Direct									$ 593,976,241	$ 561,178,447	$ 515,721,745
Assumed									473,512,866	451,515,575	423,129,271
Ceded									(326,198,234)	(310,179,267)	(282,646,219)
Net premiums earned	$ 186,150,478	$ 187,661,705	$ 185,714,110	$ 181,764,580	$ 181,059,920	$ 177,283,816	$ 175,014,872	$ 169,156,147	$ 741,290,873	$ 702,514,755	$ 656,204,797
Percentage of assumed premiums earned to net premiums earned									63.90%	64.30%	64.50%