Restricted net assets (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Restricted Cash and Cash Equivalents Items [Line Items]
Total restricted net assets in the Company's consolidated subsidiaries	$ 20,902	$ 21,096
Consolidated net assets	167,200	160,200
Cash restricted in China as a result of ongoing litigation and unfulfilled government grants
Restricted Cash and Cash Equivalents Items [Line Items]
Total restricted net assets in the Company's consolidated subsidiaries	10,896	10,936
China earnings restricted to fund statutory common reserves in China
Restricted Cash and Cash Equivalents Items [Line Items]
Total restricted net assets in the Company's consolidated subsidiaries	9,081	9,240
Loan agreements in Japan requiring local subsidiaries to maintain minimum net asset levels
Restricted Cash and Cash Equivalents Items [Line Items]
Total restricted net assets in the Company's consolidated subsidiaries	$ 925	$ 920