Net Income Per Share - Additional Information (Detail) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Earnings Per Share, Basic [Abstract]
Net income	$ 6,744,000	$ 9,355,000	$ 9,106,000	$ 7,715,000	$ 10,044,000	$ 7,269,000	$ 8,486,000	$ 6,944,000	$ 32,920,279	$ 32,743,238	$ 29,973,472
Less: Net income attributable to participating securities									(499,270)	(298,939)	(271,182)
Net income attributable to common stockholders									32,421,009	32,444,299	29,702,290
Weighted average shares outstanding used in basic per common share computations (in shares)									11,025,897	10,997,890	10,976,982
Basic EPS (in dollars per share)	$ 0.60	$ 0.83	$ 0.81	$ 0.69	$ 0.90	$ 0.66	$ 0.77	$ 0.63	$ 2.94	$ 2.95	$ 2.71
Diluted EPS
Net income attributable to common stockholders									32,421,009	32,444,299	29,702,290
Reallocation of net income attributable to participating securities									5	0	0
Net income attributable to common stockholders for diluted EPS									$ 32,421,014	$ 32,444,299	$ 29,702,290
Weighted average shares outstanding used in basic per common share computations (in shares)									11,025,897	10,997,890	10,976,982
Weighted average effect of dilutive securities: Performance share awards (in shares)									509	0	0
Weighted average shares outstanding used in diluted per common share computations (in shares)									11,026,406	10,997,890	10,976,982
Diluted EPS (in dollars per share)	$ 0.60	$ 0.83	$ 0.81	$ 0.69	$ 0.90	$ 0.66	$ 0.77	$ 0.63	$ 2.94	$ 2.95	$ 2.71