Revenue - Impact of Adoption on Statement of Operations (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net revenue	$ 43,152	$ 42,101	$ 81,459	$ 79,742
Cost of revenue	2,614	2,789	4,257	4,826
Sales and marketing	13,098	13,941	25,780	27,564
Total costs and expenses	36,586	38,626	70,408	74,996
Income from operations	6,566	3,475	11,051	4,746
Income before income taxes	6,754	2,526	11,419	3,773
Income tax expense	1,469	1,115	2,522	1,077
Net income	$ 5,285	$ 1,411	$ 8,897	$ 2,696
Basic earnings per share (in dollars per share)	$ 0.21	$ 0.06	$ 0.35	$ 0.11
Diluted earnings per share (in dollars per share)	$ 0.20	$ 0.06	$ 0.35	$ 0.11
Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net revenue		$ 42,101		$ 79,742
Cost of revenue		2,789		4,826
Sales and marketing		13,941		27,564
Total costs and expenses		38,626		74,996
Income from operations		3,475		4,746
Income before income taxes		2,526		3,773
Income tax expense		1,115		1,077
Net income		$ 1,411		$ 2,696
Basic earnings per share (in dollars per share)		$ 0.06		$ 0.11
Diluted earnings per share (in dollars per share)		$ 0.06		$ 0.11
Accounting Standards Update 2014-09 | As Reported
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net revenue		$ 42,226		$ 77,986
Cost of revenue		2,786		4,695
Sales and marketing		14,021		27,531
Total costs and expenses		38,702		74,812
Income from operations		3,524		3,174
Income before income taxes		2,575		2,201
Income tax expense		1,134		448
Net income		$ 1,441		$ 1,753
Basic earnings per share (in dollars per share)		$ 0.06		$ 0.07
Diluted earnings per share (in dollars per share)		$ 0.06		$ 0.07
Accounting Standards Update 2014-09 | Adoption of ASU 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net revenue		$ (125)		$ 1,756
Cost of revenue		3		131
Sales and marketing		(80)		33
Total costs and expenses		(76)		184
Income from operations		(49)		1,572
Income before income taxes		(49)		1,572
Income tax expense		(19)		629
Net income		$ (30)		$ 943
Basic earnings per share (in dollars per share)		$ 0.00		$ 0.04
Diluted earnings per share (in dollars per share)		$ 0.00		$ 0.04