Summary of significant accounting policies - Narrative (Details)	12 Months Ended
	Mar. 31, 2020 USD ($) shares	Mar. 31, 2019 USD ($) shares	Mar. 31, 2018 USD ($) shares	Mar. 31, 2020 ZAR (R) shares	Apr. 01, 2019 USD ($)	Apr. 01, 2018 USD ($)	May 23, 2017 ZAR (R)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred commissions	$ 3,614,000	$ 3,729,000
Deferred tax liabilities	11,436,000	9,174,000
Retained earnings	67,482,000	62,750,000
Write-down of inventory	339,000	299,000	$ 715,000
Lease liabilities	6,508,000
Goodwill impairments	0	0
Impairment of long-lived assets	$ 6,000	$ 62,000	$ 199,000
Treasury stock (in shares) | shares	53,816,750	40,000,000		53,816,750
Treasury stock acquired (in shares) | shares	13,816,750
Approved share repurchase program amount	$ 15,100,000						R 270,000,000
Shares repurchased (in shares) | shares	16,856,001	9,157,695	5,015,660
Shares repurchased	$ 9,800,000	$ 5,300,000	$ 1,400,000
Shares repurchased and delisted (in shares) | shares	3,039,251
Maximum value of shares that may still be repurchased	$ 1,900,000			R 33,500,000
Expected forfeiture rates	5.00%
Advertising costs	$ 2,000,000.0	2,000,000.0	1,900,000
Allowance for doubtful accounts	$ 3,602,000	$ 2,719,000	$ 1,482,000
Subsidiary
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Shares repurchased (in shares) | shares	13,816,750
Black Industrialists Group Property Management Company Ltd
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans	$ 400,000
Proceeds from sale and leaseback transaction	1,300,000
Gain from sale and leaseback transaction	300,000
Carrying Value
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans	500,000
Carrying Value | Black Industrialists Group Property Management Company Ltd
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans	300,000
Fair Value
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans	500,000
Fair Value | Black Industrialists Group Property Management Company Ltd
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans	$ 200,000
Minimum | Patents and trademarks
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Estimated useful life of intangible assets	3 years
Minimum | Customer relationships
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Estimated useful life of intangible assets	2 years
Minimum | Internal-use software, technology and other
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Estimated useful life of intangible assets	1 year
Maximum | Patents and trademarks
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Estimated useful life of intangible assets	20 years
Maximum | Customer relationships
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Estimated useful life of intangible assets	15 years
Maximum | Internal-use software, technology and other
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Estimated useful life of intangible assets	18 years
Subscription | Minimum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Contract service period	1 month
Subscription | Maximum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Contract service period	60 months
ASC 606 | Effect of Adoption
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred commissions						$ 3,800,000
Commission liability						600,000
Deferred revenue						400,000
Contract asset						100,000
Deferred tax liabilities						500,000
Retained earnings						2,400,000
ASC 842
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Right-of-use asset						3,000,000.0
Lease liabilities						$ 3,000,000.0
ASU 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for doubtful accounts					$ 300,000