Revenue Information and Deferred Sales Commissions - Schedule of Cumulative Effects of Changes (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Assets
Other current assets	$ 5,806		$ 5,806		$ 4,469	$ 1,145
Other assets	5,113		5,113		4,353	1,391
Liabilities
Accounts payable	26,933		26,933		18,876	18,620
Accrued expenses and other liabilities	17,561		17,561		12,897	12,790
Stockholders’ Equity
Accumulated deficit	(367,089)		(367,089)		(345,161)	(351,084)
Revenues	93,586	$ 82,440	262,497	$ 240,016
Sales and marketing	57,031	48,383	157,463	137,498
Net loss	(6,251)	$ (9,519)	(21,928)	$ (24,374)
Balances Without Adoption of New Revenue Standard
Assets
Other current assets	1,670		1,670			1,145
Other assets	1,946		1,946			1,391
Liabilities
Accounts payable	26,381		26,381			18,620
Accrued expenses and other liabilities	17,331		17,331			12,790
Stockholders’ Equity
Accumulated deficit	(373,610)		(373,610)			$ (351,084)
Revenues	93,638		261,808
Sales and marketing	56,990		157,372
Net loss	(6,158)		(22,526)
ASU 2014-09 | Adjustments Due to Adoption of New Revenue Standard
Assets
Other current assets	4,136		4,136		3,324
Other assets	3,167		3,167		2,962
Liabilities
Accounts payable	552		552		256
Accrued expenses and other liabilities	230		230		107
Stockholders’ Equity
Accumulated deficit	6,521		6,521		$ 5,923
Revenues	(52)		689
Sales and marketing	41		91
Net loss	$ 93		$ (598)