Business segment information (Details 2) - USD ($) $ in Thousands	12 Months Ended
Mar. 31, 2021	Mar. 31, 2020
Total property, plant and equipment	$ 9,500	$ 9,439
HONG KONG
Total property, plant and equipment	750	797
The PRC
Total property, plant and equipment	$ 8,750	$ 8,642

Amount of increase (decrease) in accumulated depreciation, depletion and amortization of property, plant and equipment.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229

Name:	us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipmentPeriodIncreaseDecrease
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	duration