Accumulated other comprehensive loss (Narrative) (Details) (USD $)	12 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2011	Mar. 31, 2005
Accumulated Other Comprehensive Income (Loss) [Line Items]
Deferred taxes related to adjustments included in AOCI	$ 13,406,000	$ 8,992,000	$ (216,000)
Change in pension liability and postretirement obligations, tax	12,684,000	8,178,000	(237,000)
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	12,409,000	(8,086,000)		52,000	10,006,000
Postretirement Benefit Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Change in pension liability and postretirement obligations, tax	233,000	(49,000)		(242,000)	(935,000)
Defined Benefit Postretirement Life Insurance
Accumulated Other Comprehensive Income (Loss) [Line Items]
Change in pension liability and postretirement obligations, tax	42,000	(43,000)		(47,000)	747,000
Unrealized Investment Gain
Accumulated Other Comprehensive Income (Loss) [Line Items]
Change in pension liability and postretirement obligations, tax					$ 557,000