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, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Income available to common shareholders:
- Basic and diluted	$ 13,938,895	$ 17,226,638	$ 42,925,892	$ 42,148,316
Weighted average number of shares:
- Basic	29,831,336	29,820,836	29,825,874	29,814,300
- Effect of dilutive warrants and options	141,117	71,158	146,685	182,207
- Diluted	29,972,453	29,891,994	29,972,559	29,996,507
Net income per share
- Basic	$ 0.47	$ 0.58	$ 1.44	$ 1.41
- Diluted	$ 0.46	$ 0.58	$ 1.43	$ 1.41