Summary of Significant Accounting Policies - Statement of Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net loss	$ (55,545)	$ (45,470)	$ (49,000)
Adjustments to reconcile net loss to net cash provided by operating activities	43,443
Changes in assets and liabilities:
Accounts receivable	(1,685)	(25,217)	(6,195)
Deferred contract acquisition and fulfillment costs	(12,790)	0	0
Prepaid expenses and other assets	(287)	(74)	(3,523)
Accounts payable	3,675	(2,257)	1,619
Accrued expenses	6,018	6,758	1,578
Deferred revenue	22,870	55,437	38,748
Other liabilities	367	(950)	558
Net cash provided by operating activities	6,066	$ 13,286	$ 9,112
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net loss	(56,194)
Adjustments to reconcile net loss to net cash provided by operating activities	43,125
Changes in assets and liabilities:
Accounts receivable	(1,685)
Deferred contract acquisition and fulfillment costs	0
Prepaid expenses and other assets	146
Accounts payable	3,675
Accrued expenses	6,018
Deferred revenue	10,614
Other liabilities	367
Net cash provided by operating activities	$ 6,066