Restructuring charges - Restructuring Costs (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016
Restructuring Cost and Reserve [Line Items]
Restructuring charges		$ 6,500	$ 20,292	$ 43,089
Cost of Revenue [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges			634	497
Research and Development [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges			10,092	17,197
Sales and Marketing [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges			7,047	12,064
General and Administrative [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges			2,519	$ 13,331
Non-cancelable Leases, Accelerated Depreciation and Other Charges [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges	$ 6,700
First quarter 2017 restructuring [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges	17,000
Restructuring charges			$ 16,966
First quarter 2017 restructuring [Member] | Employee Severance [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges	$ 10,312