Condensed Statements of Income (Detail) (USD $)	3 Months Ended												12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2008
General and administrative expenses	$ (2,630,168)	$ (2,059,027)	$ (1,834,465)	$ (2,097,834)	$ (2,239,462)	$ (1,498,306)	$ (1,339,062)	$ (1,573,312)	$ (1,776,457)	$ (1,192,225)	$ (1,484,893)	$ (1,293,603)	$ (8,621,494)	$ (6,650,142)	$ (5,747,178)
Other expenses:
Change in fair value of warrant													(124,000)	3,062,575	(1,448,819)
Gain on extinguishment of warrant liabilities													73,291	88,777	186,897
NET INCOME	15,793,687	17,226,638	13,446,666	11,475,012	12,727,388	13,528,083	14,366,076	12,511,986	9,855,420	9,855,705	9,898,292	8,856,558	57,942,003	53,133,533	38,465,975	11,701,879
Parent Company
General and administrative expenses													(3,397,929)	(2,267,880)	(2,911,720)
Other expenses:
Change in fair value of warrant													(124,000)	3,062,575	(1,448,819)
Gain on extinguishment of warrant liabilities													73,291	88,777	186,897
Equity in income of subsidiaries													61,390,641	52,250,061	42,639,617
NET INCOME													$ 57,942,003	$ 53,133,533	$ 38,465,975