Post-retirement Benefits - Plan asset allocation (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2018
Plan asset allocation
Fair values of the pension plan assets	$ 11,357	$ 10,055	$ 9,885
Fair Value Inputs Level1
Plan asset allocation
Fair values of the pension plan assets	427	346
Fair Value Inputs Level2
Plan asset allocation
Fair values of the pension plan assets	$ 10,930	9,709
Foreign Government Debt Securities
Plan asset allocation
Actual Allocation (as a percent)	52.00%
Fair values of the pension plan assets	$ 5,896	4,975
Foreign Government Debt Securities | Minimum
Plan asset allocation
Target Allocation (as a percent)	40.00%
Foreign Government Debt Securities | Maximum
Plan asset allocation
Target Allocation (as a percent)	50.00%
Foreign Government Debt Securities | Fair Value Inputs Level2
Plan asset allocation
Fair values of the pension plan assets	$ 5,896	4,975
Corporate Debt Securities
Plan asset allocation
Actual Allocation (as a percent)	37.00%
Fair values of the pension plan assets	$ 4,247	4,097
Corporate Debt Securities | Minimum
Plan asset allocation
Target Allocation (as a percent)	30.00%
Corporate Debt Securities | Maximum
Plan asset allocation
Target Allocation (as a percent)	40.00%
Corporate Debt Securities | Fair Value Inputs Level2
Plan asset allocation
Fair values of the pension plan assets	$ 4,247	4,097
Other Assets
Plan asset allocation
Actual Allocation (as a percent)	11.00%
Fair values of the pension plan assets	$ 1,214	983
Other Assets | Minimum
Plan asset allocation
Target Allocation (as a percent)	1.00%
Other Assets | Maximum
Plan asset allocation
Target Allocation (as a percent)	10.00%
Other Assets | Fair Value Inputs Level1
Plan asset allocation
Fair values of the pension plan assets	$ 427	346
Other Assets | Fair Value Inputs Level2
Plan asset allocation
Fair values of the pension plan assets	$ 787	$ 637