Other balance sheet items - Schedule of Prepayments and Other Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Advances to suppliers		$ 12,590	$ 6,509
Prepaid income taxes		10,042	10,935
Prepaid other taxes		11,552	10,737
Contract deposit		2,700	2,700
Other assets		1,858	3,185
Total prepayments and other assets		38,742	$ 34,066
Schedule of Equity Method Investments [Line Items]
Additional amount due that will be treated as key money if purchase not completed		$ 800
Forecast
Schedule of Equity Method Investments [Line Items]
Ownership interest planning to purchase	30.00%