Revenue Information and Deferred Sales Commissions - Schedule of Cumulative Effects of Changes (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Assets
Other current assets	$ 4,971		$ 4,971		$ 4,469	$ 1,145
Other assets	5,188		5,188		4,353	1,391
Liabilities
Accounts payable	18,445		18,445		18,876	18,620
Accrued expenses and other liabilities	14,769		14,769		12,897	12,790
Stockholders’ Equity
Accumulated deficit	(360,838)		(360,838)		(345,161)	(351,084)
Revenues	87,850	$ 81,819	168,911	$ 157,576
Sales and marketing	52,014	46,933	100,432	89,115
Net loss	(6,622)	$ (8,060)	(15,677)	$ (14,855)
Balances Without Adoption of New Revenue Standard
Assets
Other current assets						1,145
Other assets						1,391
Liabilities
Accounts payable						18,620
Accrued expenses and other liabilities						12,790
Stockholders’ Equity
Accumulated deficit						$ (351,084)
ASU 2014-09 | Balances Without Adoption of New Revenue Standard
Assets
Other current assets	897		897
Other assets	2,039		2,039
Liabilities
Accounts payable	17,952		17,952
Accrued expenses and other liabilities	14,653		14,653
Stockholders’ Equity
Accumulated deficit	(367,452)		(367,452)
Revenues	87,412		168,170
Sales and marketing	51,962		100,382
Net loss	(7,008)		(16,368)
ASU 2014-09 | Adjustments Due to Adoption of New Revenue Standard
Assets
Other current assets	4,074		4,074		3,324
Other assets	3,149		3,149		2,962
Liabilities
Accounts payable	493		493		256
Accrued expenses and other liabilities	116		116		107
Stockholders’ Equity
Accumulated deficit	6,614		6,614		$ 5,923
Revenues	438		741
Sales and marketing	52		50
Net loss	$ (386)		$ (691)