Restructuring charges - Restructuring Costs (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Sep. 30, 2018	Sep. 30, 2017
Restructuring Cost and Reserve [Line Items]
Restructuring charges			$ 21,526	$ 16,788
Cost of Revenue [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges			1,357	458
Research and Development [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges			12,032	8,406
Sales and Marketing [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges			5,042	5,960
General and Administrative [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges			3,095	$ 1,964
First quarter 2018 restructuring [Member] [Domain]
Restructuring Cost and Reserve [Line Items]
Restructuring charges	$ 16,800
Restructuring charges			16,839
First quarter 2018 restructuring [Member] [Domain] | Employee Severance [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges	$ 13,000		13,468
First quarter 2018 restructuring [Member] [Domain] | Other Restructuring [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges			3,371
First quarter 2017 restructuring [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges		$ 21,500
Restructuring charges			4,509
First quarter 2017 restructuring [Member] | Non-cancelable Leases, Accelerated Depreciation and Other Charges [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges		11,200
First quarter 2017 restructuring [Member] | Employee Severance [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges		$ 10,300	0
First quarter 2017 restructuring [Member] | Other Restructuring [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges			$ 4,509