Unearned Revenue - Changes in Unearned Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Unearned Revenue
Balance, beginning of period	$ 114,038	$ 102,879	$ 118,211	$ 105,837
Billings, excluding billings for customer arrangements with termination rights	53,047	49,477	93,838	89,800
Additions to unearned revenue upon expiration of termination rights	(4,599)	(5,062)	(9,244)	(10,062)
Recognition of revenue	(58,746)	(51,019)	(108,355)	(99,300)
Acquired unearned revenue	275		275
Balance, end of period	113,213	106,399	113,213	106,399
Increase (decrease) in total unbilled accounts receivable	179	(154)	268	(353)
Revenue Reported in Consolidated Statement of Operations	$ 58,925	$ 50,865	$ 108,623	$ 98,947