Revenue (Impact on Income Statement) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating expenses:
Sales and marketing									$ 73,092	$ 64,111	$ 57,083
Total operating expenses	$ 110,107	$ 105,842	$ 99,998	$ 111,853	$ 102,628	$ 98,798	$ 101,331	$ 102,906	427,800	405,663	329,494
Income from operations	4,958	8,129	16,510	14,787	15,452	16,048	15,801	23,131	44,384	70,432	26,067
Income before income taxes	3,641	7,058	15,545	13,871	14,009	14,299	14,121	21,541	40,115	63,970	24,831
Income tax provision	(1,738)	(4,934)	(4,621)	(2,852)	(5,020)	(5,236)	6,157	(5,484)	(14,145)	(9,583)	(8,929)
Net income	$ 1,903	$ 2,124	$ 10,924	$ 11,019	$ 8,989	$ 9,063	$ 20,278	$ 16,057	$ 25,970	$ 54,387	$ 15,902
Net income per share:
Basic (in dollars per share)	$ 0.05	$ 0.05	$ 0.27	$ 0.28	$ 0.23	$ 0.23	$ 0.54	$ 0.43	$ 0.65	$ 1.41	$ 0.44
Diluted (in dollars per share)	$ 0.05	$ 0.05	$ 0.27	$ 0.27	$ 0.23	$ 0.23	$ 0.53	$ 0.42	$ 0.64	$ 1.38	$ 0.43
Adjustments | ASU 2014-09
Operating expenses:
Sales and marketing									$ (543)
Total operating expenses									(543)
Income from operations									543
Income before income taxes									543
Income tax provision									190
Net income									$ 353
Net income per share:
Basic (in dollars per share)									$ 0.01
Diluted (in dollars per share)									$ 0.01
Balance without adoption of ASC 606
Operating expenses:
Sales and marketing									$ 72,549
Total operating expenses									427,257
Income from operations									44,927
Income before income taxes									40,658
Income tax provision									(14,335)
Net income									$ 26,323
Net income per share:
Basic (in dollars per share)									$ 0.66
Diluted (in dollars per share)									$ 0.65