CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Cash flows from operating activities:
Net income	$ 41,978,523	$ 39,058,219	$ 32,038,091
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation & amortization	8,385,413	9,177,018	8,333,089
Provision for losses on accounts receivable (Note 2)	289,666	414,042	5,229,277
Stock-based compensation expense	6,440,841	6,103,898	4,025,960
Deferred income taxes	(3,814,774)	(2,461,240)	(1,069,988)
Loss on disposal of property and equipment	98,181	174,497	51,301
Changes in operating assets and liabilities:
(Increase) in trade accounts receivable, net	(23,184,345)	(5,063,238)	(2,786,606)
(Increase) decrease in prepaid expenses & other current assets	814,581	1,631,159	(927,644)
(Increase) decrease in other assets	19,567	(156,083)	(37,203)
Increase (decrease) in accounts payable	19,730,242	2,326,103	(4,502,022)
(Decrease) in accrued expenses	(603,735)	(3,450,041)	(586,798)
Increase (decrease) in other current liabilities	(305,277)	2,627,655	222,786
Increase in other long-term liabilities	909,355	900,344	863,096
Net cash provided by operating activities	50,758,238	51,282,333	40,853,339
Cash flows from investing activities:
Purchases of property and equipment	(4,453,495)	(5,380,800)	(6,717,034)
Proceeds from sale of property and equipment	3,600	400	16,153
Capitalized internal use software development costs	(991,543)	(1,478,623)	(1,489,286)
Net cash used in investing activities	(5,441,438)	(6,859,023)	(8,190,167)
Cash flows from financing activities:
Cash dividends on common stock		(32,977,016)
Cash restricted for payment of dividend	(36,455,955)		(22,982,447)
Proceeds from employee common stock purchases	1,131,389	1,106,777	904,471
Tax deductions related to stock-based compensation	412,617	1,184,860	1,302,005
Net cash used in financing activities	(34,911,949)	(30,685,379)	(20,775,971)
Net increase in cash and cash equivalents	10,404,851	13,737,931	11,887,201
Cash and cash equivalents, beginning of period	87,983,398	74,245,467	62,358,266
Cash and cash equivalents, end of period	98,388,249	87,983,398	74,245,467
Non-cash investing activities:
Capital expenditures accrued but not yet paid	678	102,088	185,001
Cash payments:
Income taxes paid	$ 27,222,391	25,059,316	$ 15,939,214
Cash dividends on common stock previously restricted for payment of dividend		$ 22,982,447