Unearned Revenue - Changes in Unearned Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Unearned Revenue
Balance, beginning of period	$ 106,399	$ 82,934	$ 105,837	$ 77,022
Billings, excluding billings for customer arrangements with termination rights	46,246	55,819	135,904	144,107
Additions to unearned revenue upon expiration of termination rights	(5,056)	(4,385)	(15,118)	(12,741)
Recognition of revenue, net of change in unbilled accounts receivable	(50,900)	(50,164)	(150,058)	(140,896)
Revenue billed as of the end of period	106,801	92,974	106,801	92,974
Increase (decrease) in total unbilled accounts receivable	1,301	(913)	1,090	(1,827)
Revenue Reported in Condensed Consolidated Statement of Operations	$ 52,201	$ 49,251	$ 151,148	$ 139,069