Reconciliation of Net Income and Weighted Average Shares Used in Computation of Basic and Diluted Earnings Per Share (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Income available to common shareholders:
- Basic and diluted	$ 17,226,638	$ 13,528,083	$ 42,148,316	$ 40,406,145
Weighted average number of shares:
- Basic	29,820,836	29,768,403	29,814,300	29,833,041
- Effect of dilutive warrants and options	71,158	208,299	182,207	291,750
- Diluted	29,891,994	29,976,702	29,996,507	30,124,791
Net income per share
- Basic	$ 0.58	$ 0.45	$ 1.41	$ 1.35
- Diluted	$ 0.58	$ 0.45	$ 1.41	$ 1.34