Condensed Balance Sheets (Detail) (USD $)	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Current assets:
Cash and cash equivalents	$ 203,406,220	$ 144,300,290	$ 105,637,627	$ 90,609,340
Other receivables	337,189	559,955
Prepayments	418,502	406,026
Total current assets	294,089,943	227,989,786
Total assets	365,533,456	293,911,897
Current liabilities:
Warrant liabilities	33,262	354,000
Total current liabilities	19,533,646	14,979,702
Stockholders’ equity
Preferred stock: $0.0001 par value, 10,000,000 shares authorized, none issued and outstanding	0	0
Common stock, $0.0001 par value: 75,000,000 shares authorized, 30,295,955 shares issued and 30,031,908 shares outstanding as of December 31, 2013 (2012: 30,084,883 shares issued and 29,820,836 shares outstanding), respectively	3,029	3,008
Additional paid-in capital	80,627,496	79,257,921
Treasury stock, at cost, 264,047 shares and 264,047 as of December 31, 2013 and 2012, respectively	(2,126,597)	(2,126,597)
Retained earnings	222,165,580	170,163,120
Accumulated other comprehensive income	26,606,466	17,067,897
Total stockholders' equity	345,999,810	278,932,195	219,636,640	153,157,623
Total liabilities and stockholders' equity	365,533,456	293,911,897
Parent
Current assets:
Cash and cash equivalents	43,010	64,015	0	0
Other receivables	16,333	15,843
Prepayments	136,500	162,000
Total current assets	195,843	241,858
Investments in subsidiaries	345,837,229	279,044,337
Total assets	346,033,072	279,286,195
Current liabilities:
Warrant liabilities	33,262	354,000
Total current liabilities	33,262	354,000
Stockholders’ equity
Preferred stock: $0.0001 par value, 10,000,000 shares authorized, none issued and outstanding	0	0
Common stock, $0.0001 par value: 75,000,000 shares authorized, 30,295,955 shares issued and 30,031,908 shares outstanding as of December 31, 2013 (2012: 30,084,883 shares issued and 29,820,836 shares outstanding), respectively	3,029	3,008
Additional paid-in capital	80,627,496	79,257,921
Treasury stock, at cost, 264,047 shares and 264,047 as of December 31, 2013 and 2012, respectively	(2,126,597)	(2,126,597)
Retained earnings	240,889,416	184,729,966
Accumulated other comprehensive income	26,606,466	17,067,897
Total stockholders' equity	345,999,810	278,932,195
Total liabilities and stockholders' equity	$ 346,033,072	$ 279,286,195