Restructuring charges - Restructuring Costs (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Restructuring Cost and Reserve [Line Items]
Restructuring charges			$ 1,454	$ 22,743	$ 20,292
Cost of Revenue [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges			54	1,379	634
Research and Development [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges			585	12,794	10,092
Selling and Marketing Expense [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges			314	5,291	7,047
General and Administrative [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges			501	3,279	2,519
First quarter 2018 restructuring [Member] [Domain]
Restructuring Cost and Reserve [Line Items]
Restructuring charges	$ 17,800
Restructuring charges			8	17,793
First quarter 2017 restructuring [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges		$ 23,100
Restructuring charges			$ 1,395	4,783	16,966
First quarter 2017 restructuring [Member] | Non-cancelable Leases, Accelerated Depreciation and Other Charges [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges		$ 12,800
First quarter 2017 restructuring [Member] | Employee Severance [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges				0	10,312
First quarter 2017 restructuring [Member] | Other Restructuring [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges				$ 4,783	$ 6,654