Summary of Significant Accounting Policies - ASC 606 - Impact - Balance sheet (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Oct. 01, 2018	Sep. 30, 2018
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Prepaid expenses and other current assets	$ 42,063	$ 32,091	$ 31,741
Other assets	22,304	6,777	5,294
Deferred revenue	31,091	28,734	25,884
Accumulated deficit	(987,725)	(994,933)	$ (994,074)
Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Prepaid expenses and other current assets		350
Other assets		1,483
Deferred revenue		2,850
Accumulated deficit		$ (859)
Calculated under Revenue Guidance in Effect before Topic 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Prepaid expenses and other current assets	37,292
Other assets	21,025
Deferred revenue	28,944
Accumulated deficit	(991,628)
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Prepaid expenses and other current assets	4,771
Other assets	1,279
Deferred revenue	2,147
Accumulated deficit	$ 3,903