Reinsurance - Amounts Represent Effect of Reinsurance on Premiums Earned (Detail) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reinsurance Disclosures [Abstract]
Direct									$ 561,178,447	$ 515,721,745	$ 480,210,534
Assumed									451,515,575	423,129,271	397,142,483
Ceded									(310,179,267)	(282,646,219)	(271,712,289)
Net premiums earned	$ 181,059,920	$ 177,283,816	$ 175,014,872	$ 169,156,147	$ 168,977,030	$ 166,809,851	$ 161,942,637	$ 158,475,279	$ 702,514,755	$ 656,204,797	$ 605,640,728
Percentage of assumed premiums earned to net premiums earned									64.30%	64.50%	65.60%