Accumulated other comprehensive loss (Narrative) (Details) - USD ($)	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2013	Mar. 31, 2005
Accumulated Other Comprehensive Income (Loss) [Line Items]
Deferred taxes related to adjustments included in AOCI	$ (6,848,000)	$ (5,579,000)	$ 4,753,000
Change in pension liability and postretirement obligations, tax	5,205,000	6,442,000	(4,197,000)
Amount of charge in the minimum pension liability component of other comprehensive income					$ 534,000
Unrealized Investment Gain
Accumulated Other Comprehensive Income (Loss) [Line Items]
Change in pension liability and postretirement obligations, tax	88,000			$ (557,000)
Pension Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Change in pension liability and postretirement obligations, tax	4,981,000	6,043,000	(4,635,000)	(10,006,000)
Postretirement Benefit Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Change in pension liability and postretirement obligations, tax	153,000	317,000	372,000	(935,000)
Defined Benefit Postretirement Life Insurance
Accumulated Other Comprehensive Income (Loss) [Line Items]
Change in pension liability and postretirement obligations, tax	70,000	$ 82,000	$ 66,000	$ (747,000)
Pension Plans | Pension Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Change in pension liability and postretirement obligations, tax	9,477,000
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	$ 194,000