Revenue (Impact on Income Statement) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Operating expenses:
Sales and marketing	$ 17,920	$ 16,385	$ 54,779	$ 47,192
Total operating expenses	105,842	98,798	317,694	303,035
Income from operations	8,129	16,048	39,426	54,980
Income before income taxes	7,058	14,299	36,474	49,961
Income tax provision	(4,934)	(5,236)	(12,407)	(4,563)
Net income	$ 2,124	$ 9,063	$ 24,067	$ 45,398
Net income per share:
Basic (in dollars per share)	$ 0.05	$ 0.23	$ 0.60	$ 1.19
Diluted (in dollars per share)	$ 0.05	$ 0.23	$ 0.59	$ 1.16
Balances without adoption of ASC 606
Operating expenses:
Sales and marketing	$ 17,851		$ 55,004
Total operating expenses	105,773		317,919
Income from operations	8,198		39,201
Income before income taxes	7,127		36,249
Income tax provision	(4,982)		(12,331)
Net income	$ 2,145		$ 23,918
Net income per share:
Basic (in dollars per share)	$ 0.05		$ 0.60
Diluted (in dollars per share)	$ 0.05		$ 0.59
Adjustments | ASU 2014-09
Operating expenses:
Sales and marketing	$ (69)		$ 225
Total operating expenses	(69)		225
Income from operations	69		(225)
Income before income taxes	69		(225)
Income tax provision	(48)		76
Net income	$ 21		$ (149)