Summary of Significant Accounting Policies - Basic and diluted earnings per share (Details 7) (USD $)	3 Months Ended								12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2011
Accounting Policies [Abstract]
Numerator for earnings per share - net income	$ 4,787,296	$ 4,596,056	$ 5,150,138	$ 5,407,456	$ 6,001,993	$ 5,378,843	$ 5,532,365	$ 5,308,558	$ 19,940,946	$ 22,221,759	$ 16,768,294
Denominator:
Denominator for basic earnings per share - weighted average shares									11,977,174	11,747,160	11,607,341
Effect of dilutive securities:
Stock options and other share awards									241,242	285,971	286,177
Denominator for diluted earnings per share									12,218,416	12,033,131	11,893,518
Earnings per share - basic (in dollars per share)	$ 0.40	$ 0.38	$ 0.43	$ 0.45	$ 0.51	$ 0.46	$ 0.47	$ 0.46	$ 1.66	$ 1.89	$ 1.44
Earnings per share - diluted (in dollars per share)	$ 0.39	$ 0.38	$ 0.42	$ 0.44	$ 0.50	$ 0.45	$ 0.46	$ 0.44	$ 1.63	$ 1.85	$ 1.41