Commitments and contingencies (Narrative) (Details) $ in Thousands		1 Months Ended	3 Months Ended
	Nov. 22, 2017 patent	Aug. 31, 2017 patent	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	[1]
Operating Leased Assets [Line Items]
Decrease in deferred rent			$ (35)	$ (191)
Tenant improvement allowance recognition due to lease termination			0	$ 582
Avaya Sublease
Operating Leased Assets [Line Items]
Decrease in deferred rent			538
Tenant improvement allowance recognition due to lease termination			582
Indemnification Agreement
Operating Leased Assets [Line Items]
Number of patents allegedly infringed the PTO reexamined | patent	15	5
Litigation settlement			250
General and Administrative Expense | Indemnification Agreement
Operating Leased Assets [Line Items]
Litigation settlement			$ 250

[1] See Note 1 for a summary of adjustments.