Pension and Postretirement Benefits Postretirement Health and Life Insurance Benefits (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Defined Benefit Plan Disclosure [Line Items]
Contributions by employer			$ 2,916
Additional contributions to the plans are expected during the rest of current fiscal year			3,544
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Service cost	$ 120	$ 527	240	$ 1,029
Interest expense	1,176	1,462	2,352	2,924
Amortization of prior service cost	(10)	(41)	(20)	(83)
Actuarial loss	524	850	1,048	1,699
Net periodic pension cost (benefit)	427	1,325	854	2,626
Health and Life Insurance Benefit Plans
Defined Benefit Plan Disclosure [Line Items]
Contributions by employer			152
Additional contributions to the plans are expected during the rest of current fiscal year			222
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Service cost	3	10	6	20
Interest expense	67	110	134	221
Amortization of prior service cost	(177)	(3)	(354)	(6)
Actuarial loss	104	112	208	224
Net periodic pension cost (benefit)	$ (3)	$ 229	$ (6)	$ 459