Pension and Postretirement Benefits Postretirement Health and Life Insurance Benefits (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Defined Benefit Plan Disclosure [Line Items]
Contributions by employer			$ 4,117
Additional contributions to the plans are expected during the rest of fiscal 2013			4,562
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Service cost	$ 527	$ 510	1,029	$ 1,020
Interest expense	1,462	1,694	2,924	3,387
Amortization of prior service cost	41	49	83	97
Actuarial loss	850	557	1,699	1,114
Net periodic pension cost (benefit)	1,325	1,133	2,626	2,264
Health and Life Insurance Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Contributions by employer			326
Additional contributions to the plans are expected during the rest of fiscal 2013			404
Negative plan amendment	4,461		4,461
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Service cost	10	10	20	21
Interest expense	110	134	221	268
Amortization of prior service cost	(3)	(303)	(6)	(606)
Actuarial loss	112	111	224	222
Net periodic pension cost (benefit)	$ 229	$ (48)	$ 459	$ (95)