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Stock-based compensation plan - Narrative (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Mar. 31, 2021
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Stock-based compensation expense $ 0.3 $ 0.3 $ 0.7 $ 0.6  
SARs          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Unrecognized compensation cost $ 1.4   $ 1.4    
Expected period for recognition of unvested awards     3 years 9 months 18 days    
Number of shares outstanding 38,875,000   38,875,000   40,567,917
RSUs          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Unrecognized compensation cost $ 0.3   $ 0.3    
Expected period for recognition of unvested awards     1 year 6 months    
Number of shares outstanding 2,000,000   2,000,000   2,000,000
Number of shares unvested 2,000,000   2,000,000   2,000,000
Estimated forfeiture rate     5.00%    
LTIP          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Number of shares reserved for future issuance (in shares) 47,090,000   47,090,000