Reconciliation of Net Income and Weighted Average Shares Used in Computation of Basic and Diluted Earnings Per Share (Detail) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Income available to common shareholders:
- Basic and diluted	$ 13,446,666	$ 14,366,076	$ 24,921,678	$ 26,878,062
Weighted average number of shares:
- Basic	29,820,836	29,872,934	29,810,996	29,865,896
- Effect of dilutive warrants and options	197,149	238,208	238,340	334,191
- Diluted	30,017,985	30,111,142	30,049,336	30,200,087
Net income per share
- Basic	$ 0.45	$ 0.48	$ 0.84	$ 0.90
- Diluted	$ 0.45	$ 0.48	$ 0.83	$ 0.89