Accumulated other comprehensive loss (Narrative) (Details) - USD ($)	12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2013	Mar. 31, 2005
Accumulated Other Comprehensive Income (Loss) [Line Items]
Deferred taxes related to adjustments included in AOCI	$ (5,579,000)	$ 4,753,000	$ 13,406,000
Change in pension liability and postretirement obligations, tax	(6,442,000)	4,197,000	12,684,000
Amount of charge in the minimum pension liability component of other comprehensive income					$ 534,000
Pension Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Change in pension liability and postretirement obligations, tax	(6,043,000)	4,635,000	12,409,000	$ 10,006,000
Postretirement Benefit Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Change in pension liability and postretirement obligations, tax	(317,000)	(372,000)	233,000	935,000
Defined Benefit Postretirement Life Insurance
Accumulated Other Comprehensive Income (Loss) [Line Items]
Change in pension liability and postretirement obligations, tax	$ (82,000)	$ (66,000)	$ 42,000	747,000
Unrealized Investment Gain
Accumulated Other Comprehensive Income (Loss) [Line Items]
Change in pension liability and postretirement obligations, tax				$ 557,000