Condensed Balance Sheets (Detail) (USD $)	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Current assets
Cash and cash equivalents	$ 144,300,290	$ 105,637,627	$ 90,609,340	$ 34,614,838
Other receivable	559,955	1,882,864
Prepayments	406,026	405,034
Total current assets	227,989,786	176,593,796
Total assets	293,911,897	238,289,113
Current liabilities:
Dividend payable	0	992,846
Warrant liabilities	354,000	615,000
Total current liabilities	14,979,702	18,652,473
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, 30,084,883 shares issued and 29,820,836 shares outstanding as of December 31, 2012 (2011: 30,036,481 shares issued and 29,772,434 shares outstanding), respectively	3,008	3,003
Additional paid-in capital	79,257,921	78,564,128
Treasury stock, at cost, 264,047 shares and 264,047 as of December 31, 2012 and 2011, respectively	(2,126,597)	(2,126,597)
Retained earnings	170,163,120	116,369,487
Accumulated other comprehensive income	17,067,897	16,408,143
Total stockholders' equity	278,932,195	219,636,640	153,157,623	67,505,759
Total liabilities and stockholders' equity	293,911,897	238,289,113
Parent Company
Current assets
Cash and cash equivalents	64,015	0	0	0
Other receivable	15,843	0
Prepayments	162,000	123,250
Total current assets	241,858	123,250
Investments in subsidiaries	279,044,337	221,121,236
Total assets	279,286,195	221,244,486
Current liabilities:
Dividend payable	0	992,846
Warrant liabilities	354,000	615,000
Total current liabilities	354,000	1,607,846
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, 30,084,883 shares issued and 29,820,836 shares outstanding as of December 31, 2012 (2011: 30,036,481 shares issued and 29,772,434 shares outstanding), respectively	3,008	3,003
Additional paid-in capital	79,257,921	78,564,128
Treasury stock, at cost, 264,047 shares and 264,047 as of December 31, 2012 and 2011, respectively	(2,126,597)	(2,126,597)
Retained earnings	184,729,966	126,792,606
Accumulated other comprehensive income	17,067,897	16,403,500
Total stockholders' equity	278,932,195	219,636,640
Total liabilities and stockholders' equity	$ 279,286,195	$ 221,244,486