Accumulated other comprehensive loss (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2015	Mar. 31, 2005
Accumulated Other Comprehensive Income (Loss) [Line Items]
Deferred taxes related to adjustments included in AOCI	$ 7,445	$ 2,566	$ (6,848)
Change in pension liability and postretirement obligations, tax	(8,010)	(2,097)	5,205
Amount of charge in the minimum pension liability component of other comprehensive income					$ 534
Pension Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Change in pension liability and postretirement obligations, tax	(8,062)	(2,242)	4,981	$ (10,006)
Postretirement Benefit Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Change in pension liability and postretirement obligations, tax	35	126	153	(935)
Defined Benefit Postretirement Life Insurance
Accumulated Other Comprehensive Income (Loss) [Line Items]
Change in pension liability and postretirement obligations, tax	17	$ 18	$ 70	$ (747)
Pension Plans \| Pension Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Change in pension liability and postretirement obligations, tax	(9,477)
Defined Benefit Postretirement Life Insurance \| Defined Benefit Postretirement Life Insurance
Accumulated Other Comprehensive Income (Loss) [Line Items]
Change in pension liability and postretirement obligations, tax	$ (194)