Reconciliation of Total Segment Operating Income to Total Consolidated Income Before Income Taxes (Detail) (USD $)	3 Months Ended								12 Months Ended
	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, 2011	Dec. 31, 2010	Dec. 31, 2009
Segment Reporting Information [Line Items]
Operating income (loss)	$ 9,620,512	$ 10,979,966	$ 9,442,917	$ 8,464,677	$ 8,032,180	$ 9,247,948	$ 6,551,651	$ 5,566,335	$ 38,508,072	$ 29,398,114	$ 22,020,844
Other income (expense), net	(28,831)	(9,727)	615	3,123	(9,419)	299	(1,353)	916	(34,820)	(9,557)	51,705
Income before income taxes	9,591,681	10,970,239	9,443,532	8,467,800	8,022,761	9,248,247	6,550,298	5,567,251	38,473,252	29,388,557	22,072,549
Total segment
Segment Reporting Information [Line Items]
Operating income (loss)									60,912,777	51,395,682	42,392,114
Other Reconciling Items
Segment Reporting Information [Line Items]
Operating income (loss)									$ (22,404,705)	$ (21,997,568)	$ (20,371,270)