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, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2008
Income available to common shareholders:
- Basic and diluted	$ 15,793,687	$ 17,226,638	$ 13,446,666	$ 11,475,012	$ 12,727,388	$ 13,528,083	$ 14,366,076	$ 12,511,986	$ 9,855,420	$ 9,855,705	$ 9,898,292	$ 8,856,558	$ 57,942,003	$ 53,133,533	$ 38,465,975	$ 11,701,879
Weighted average number of shares:
- Basic													29,815,943	29,816,871	27,956,451
- Effect of dilutive securities - warrants, options and convertible preferred stock													149,868	259,259	714,912
- Diluted													29,965,811	30,076,130	28,671,363
Net income per share
- Basic	$ 0.52	$ 0.58	$ 0.45	$ 0.39	$ 0.43	$ 0.45	$ 0.48	$ 0.42	$ 0.34	$ 0.34	$ 0.35	$ 0.35	$ 1.94	$ 1.78	$ 1.38
- Diluted	$ 0.52	$ 0.58	$ 0.45	$ 0.38	$ 0.43	$ 0.45	$ 0.48	$ 0.41	$ 0.33	$ 0.33	$ 0.34	$ 0.34	$ 1.93	$ 1.77	$ 1.34