Consolidated Balance Sheets (Parenthetical) - USD ($)	Jun. 30, 2019	Dec. 31, 2018
Statement Of Financial Position [Abstract]
Property, buildings and equipment, accumulated depreciation	$ 47,399,762	$ 43,060,083
Finite-lived intangible assets, accumulated amortization	$ 354,718	$ 324,634
Preferred stock, par value (usd per share)	$ 1	$ 1
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares issued	0	0
Common stock, par value (usd per share)	$ 0.10	$ 0.10
Common stock, shares authorized	40,000,000	40,000,000
Common stock, shares issued	27,813,772	27,813,772
Common stock, shares outstanding	24,522,534	24,590,243
Treasury stock, shares	3,291,238	3,223,529