CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities:
Net income	$ 51,614	$ 55,833	$ 41,979
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation & amortization	6,929	6,749	8,385
Stock-based compensation expense	5,464	5,997	6,441
Deferred income taxes	1,640	(886)	(1,918)
Provision for losses on accounts receivable	552	142	290
Loss on disposal of property and equipment	49	24	98
Excess tax benefits related to stock-based compensation	0	590	413
Changes in operating assets and liabilities:
(Increase) in trade accounts receivable, net	(21,769)	(2,501)	(23,184)
Decrease (increase) in prepaid expenses & other current assets	2,191	(2,449)	(1,082)
(Increase) decrease in other assets	(1,509)	(51)	20
Increase in accounts payable	15,669	12,119	19,730
Increase in accrued expenses	2,251	2,136	1,234
Increase (decrease) in other current liabilities	522	553	(305)
Increase in other long-term liabilities	1,233	2,903	909
Net cash provided by operating activities	64,836	81,159	53,010
Cash flows from investing activities:
Purchases of property and equipment	(4,771)	(5,646)	(4,454)
Proceeds from sale of property and equipment	7	8	4
Capitalized internal use software development costs	(3,565)	(2,576)	(992)
Net cash used in investing activities	(8,329)	(8,214)	(5,442)
Cash flows from financing activities:
Cash dividends on common stock	(21,393)	(43,301)	0
Cash restricted for payment of dividend	0	0	(36,456)
Proceeds from employee common stock purchases	1,330	1,114	1,131
Tax withholdings related to stock-based compensation awards	(2,676)	(2,137)	(1,838)
Net cash used in financing activities	(22,739)	(44,324)	(37,163)
Net increase in cash	33,768	28,621	10,405
Cash, beginning of period	127,009	98,388	87,983
Cash, end of period	160,777	127,009	98,388
Non-cash investing activities:
Capital expenditures accrued but not yet paid	855	273	0
Cash payments:
Income taxes paid, net of refunds	21,303	19,847	27,222
Cash dividends on common stock previously restricted for payment of dividend	$ 0	$ 36,456	$ 0