Income tax provision (benefit) (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Current:
Federal	$ 19,219,251	$ 9,161,855
State	5,336,885	2,664,336
Current Income Tax Expense (Benefit)	24,556,136	11,826,191
Deferred:
Federal	(18,718,113)	(2,199,180)
State	(1,951,238)	(810,599)
Deferred Income Tax Expense (Benefit)	(20,675,807)	(3,009,779)
Total provision for income taxes	$ 3,886,785	$ 8,816,412