Revenue - Impact of Adoption on Statement of Operations (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
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net revenue	$ 43,057	$ 40,609	$ 124,516	$ 120,351
Cost of revenue	1,793	2,307	6,050	7,133
Sales and marketing	13,187	12,197	38,967	39,761
Total costs and expenses	38,025	35,009	108,433	110,005
Income from operations	5,032	5,600	16,083	10,346
Income before income taxes	5,254	5,728	16,673	9,501
Income tax expense	960	2,148	3,482	3,225
Net income	$ 4,294	$ 3,580	$ 13,191	$ 6,276
Basic earnings per share (in dollars per share)	$ 0.17	$ 0.14	$ 0.52	$ 0.25
Diluted earnings per share (in dollars per share)	$ 0.17	$ 0.14	$ 0.51	$ 0.25
Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net revenue		$ 40,609		$ 120,351
Cost of revenue		2,307		7,133
Sales and marketing		12,197		39,761
Total costs and expenses		35,009		110,005
Income from operations		5,600		10,346
Income before income taxes		5,728		9,501
Income tax expense		2,148		3,225
Net income		$ 3,580		$ 6,276
Basic earnings per share (in dollars per share)		$ 0.14		$ 0.25
Diluted earnings per share (in dollars per share)		$ 0.14		$ 0.25
Accounting Standards Update 2014-09 | As Reported
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net revenue		$ 40,237		$ 118,223
Cost of revenue		2,314		7,009
Sales and marketing		12,220		39,732
Total costs and expenses		35,040		109,852
Income from operations		5,197		8,371
Income before income taxes		5,325		7,526
Income tax expense		1,984		2,432
Net income		$ 3,341		$ 5,094
Basic earnings per share (in dollars per share)		$ 0.13		$ 0.20
Diluted earnings per share (in dollars per share)		$ 0.13		$ 0.20
Accounting Standards Update 2014-09 | Adoption of ASU 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net revenue		$ 372		$ 2,128
Cost of revenue		(7)		124
Sales and marketing		(23)		29
Total costs and expenses		(31)		153
Income from operations		403		1,975
Income before income taxes		403		1,975
Income tax expense		164		793
Net income		$ 239		$ 1,182
Basic earnings per share (in dollars per share)		$ 0.01		$ 0.05
Diluted earnings per share (in dollars per share)		$ 0.01		$ 0.05