Revenue from Contracts with Customers - Balance Sheet (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Jan. 01, 2018	Dec. 31, 2017	Jun. 30, 2017
Assets
Cash and cash equivalents	$ 100,731	$ 51,562	$ 51,562	$ 53,068
Short-term investments	13,983	39,178	39,178
Accounts receivable, net	48,476	73,661	73,661
Deferred contract acquisition and fulfillment costs, current portion	9,481	7,844	0
Prepaid expenses and other current assets	13,212	8,907	8,877
Long-term investments	3,929	1,102	1,102
Property and equipment, net	11,184	8,589	8,589
Goodwill	83,164	83,164	83,164
Intangible assets, net	16,002	16,640	16,640
Deferred contract acquisition and fulfillment costs, non-current portion	22,214	19,321	0
Other assets	1,395	1,363	1,363
Total assets	323,771	311,331	284,136
Liabilities
Accounts payable	4,612	2,240	2,240
Accrued expenses	21,832	29,728	29,728
Deferred revenue, current portion	153,634	142,020	155,811
Other current liabilities	1,079	1,706	1,706
Deferred revenue, non-current portion	70,766	83,373	68,689
Other long-term liabilities	2,305	2,238	1,809
Total liabilities	254,228	261,305	259,983
Common stock	467	441	441
Treasury stock	(4,764)	(4,764)	(4,764)
Additional paid-in-capital	513,598	463,428	463,428
Accumulated other comprehensive loss	(24)	(39)	(39)
Accumulated deficit	(439,734)	(409,040)	(434,913)
Total stockholders’ equity	69,543	50,026	24,153
Total liabilities and stockholders’ equity	323,771	$ 311,331	284,136
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Assets
Cash and cash equivalents	100,731		51,562
Short-term investments	13,983		39,178
Accounts receivable, net	48,476		73,661
Deferred contract acquisition and fulfillment costs, current portion	0		0
Prepaid expenses and other current assets	12,995		8,877
Long-term investments	3,929		1,102
Property and equipment, net	11,184		8,589
Goodwill	83,164		83,164
Intangible assets, net	16,002		16,640
Deferred contract acquisition and fulfillment costs, non-current portion	0		0
Other assets	1,395		1,363
Total assets	291,859		284,136
Liabilities
Accounts payable	4,612		2,240
Accrued expenses	21,832		29,728
Deferred revenue, current portion	163,121		155,811
Other current liabilities	1,079		1,706
Deferred revenue, non-current portion	53,378		68,689
Other long-term liabilities	1,876		1,809
Total liabilities	245,898		259,983
Common stock	467		441
Treasury stock	(4,764)		(4,764)
Additional paid-in-capital	513,598		463,428
Accumulated other comprehensive loss	(24)		(39)
Accumulated deficit	(463,316)		(434,913)
Total stockholders’ equity	45,961		24,153
Total liabilities and stockholders’ equity	$ 291,859		$ 284,136