Revenue from Contracts with Customers - Balance Sheet (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017	Mar. 31, 2017
Assets
Cash and cash equivalents	$ 99,646	$ 51,562	$ 51,562	$ 50,662
Short-term investments	29,630	39,178	39,178
Accounts receivable, net	38,718	73,661	73,661
Deferred contract acquisition and fulfillment costs, current portion	8,583	7,844	0
Prepaid expenses and other current assets	12,232	8,907	8,877
Long-term investments	1,096	1,102	1,102
Property and equipment, net	9,238	8,589	8,589
Goodwill	83,164	83,164	83,164
Intangible assets, net	16,316	16,640	16,640
Deferred contract acquisition and fulfillment costs, non-current portion	20,295	19,321	0
Other assets	1,552	1,363	1,363
Total assets	320,470	311,331	284,136
Liabilities
Accounts payable	5,669	2,240	2,240
Accrued expenses	18,372	29,728	29,728
Deferred revenue, current portion	140,448	142,020	155,811
Other current liabilities	1,702	1,706	1,706
Deferred revenue, non-current portion	78,450	83,373	68,689
Other long-term liabilities	1,907	2,238	1,809
Total liabilities	246,548	261,305	259,983
Common stock	462	441	441
Treasury stock	(4,764)	(4,764)	(4,764)
Additional paid-in-capital	503,669	463,428	463,428
Accumulated other comprehensive loss	(44)	(39)	(39)
Accumulated deficit	(425,401)	(409,040)	(434,913)
Total stockholders’ equity	73,922	50,026	24,153
Total liabilities and stockholders’ equity	320,470	$ 311,331	284,136
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Assets
Cash and cash equivalents	99,646		51,562
Short-term investments	29,630		39,178
Accounts receivable, net	38,718		73,661
Deferred contract acquisition and fulfillment costs, current portion	0		0
Prepaid expenses and other current assets	11,949		8,877
Long-term investments	1,096		1,102
Property and equipment, net	9,238		8,589
Goodwill	83,164		83,164
Intangible assets, net	16,316		16,640
Deferred contract acquisition and fulfillment costs, non-current portion	0		0
Other assets	1,552		1,363
Total assets	291,309		284,136
Liabilities
Accounts payable	5,669		2,240
Accrued expenses	18,372		29,728
Deferred revenue, current portion	152,336		155,811
Other current liabilities	1,702		1,706
Deferred revenue, non-current portion	61,730		68,689
Other long-term liabilities	1,478		1,809
Total liabilities	241,287		259,983
Common stock	462		441
Treasury stock	(4,764)		(4,764)
Additional paid-in-capital	503,669		463,428
Accumulated other comprehensive loss	(44)		(39)
Accumulated deficit	(449,301)		(434,913)
Total stockholders’ equity	50,022		24,153
Total liabilities and stockholders’ equity	$ 291,309		$ 284,136