Income taxes (Deferred Tax Assets) (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Mar. 31, 2016
Current deferred tax assets
Net operating loss carryforwards	$ 0	$ 2,739
Inventory valuation	0	14
Reserves and allowances	0	2,740
Net current deferred tax assets	0	5,493
Net operating loss carryforwards	36,427	38,449
Research and development and other credit carryforwards	8,614	7,106
Stock-based compensation	6,942	5,577
Reserves and allowances	3,266	0
Fixed assets and intangibles	(3,688)	(6,160)
Net non-current deferred tax assets	51,561	44,972
Valuation allowance	(2,934)	(3,760)
Total	$ 48,627	$ 46,705