Restricted net assets (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021	Mar. 31, 2021	Dec. 31, 2020
Restricted Cash and Cash Equivalents Items [Line Items]
Total restricted net assets in the Company's consolidated subsidiaries	$ 11,362	$ 11,381
Stockholders' equity	151,511	152,961	$ 169,658	$ 180,407
Cash restricted in China due to unpaid employee benefits and unfulfilled government grants
Restricted Cash and Cash Equivalents Items [Line Items]
Total restricted net assets in the Company's consolidated subsidiaries	49	49
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	10,493	10,463
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	$ 820	$ 869