Reconciliation of Net Income and Weighted Average Shares Used in Computation of Basic and Diluted Earnings Per Share (Detail) (USD $)	3 Months Ended												12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2008
Income available to common stockholders:
- Basic and diluted	$ 13,233,558	$ 13,938,895	$ 15,675,810	$ 13,311,187	$ 15,793,687	$ 17,226,638	$ 13,446,666	$ 11,475,012	$ 12,727,388	$ 13,528,083	$ 14,366,076	$ 12,511,986	$ 56,159,450	$ 57,942,003	$ 53,133,533	$ 11,701,879
Weighted average number of shares:
- Basic													29,858,880	29,815,943	29,816,871
- Effect of dilutive securities - warrants and options													137,953	149,868	259,259
- Diluted													29,996,833	29,965,811	30,076,130
Net income per share
- Basic	$ 0.44	$ 0.47	$ 0.53	$ 0.45	$ 0.52	$ 0.58	$ 0.45	$ 0.39	$ 0.43	$ 0.45	$ 0.48	$ 0.42	$ 1.88	$ 1.94	$ 1.78
- Diluted	$ 0.44	$ 0.46	$ 0.52	$ 0.44	$ 0.52	$ 0.58	$ 0.45	$ 0.38	$ 0.43	$ 0.45	$ 0.48	$ 0.41	$ 1.87	$ 1.93	$ 1.77