Summary of Significant Accounting Policies (Details 3) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2011
Reconciliation of weighted-average outstanding shares used in calculation of basic earnings per share to diluted earnings per share
Basic EPS shares outstanding (in shares)	32,447	32,874	33,275
Dilutive effect of stock-based awards (in shares)	161	238	340
Diluted EPS shares outstanding (in shares)	32,608	33,112	33,615
Shares excluded from the calculation of diluted EPS because the option exercise/threshold price was greater than the average market price of the common shares (in shares)	5	5	5
Components of accumulated OCI, net of income taxes
Derivatives	$ (2,192)	$ 3,416
Pension and other postretirement benefit liabilities	(826,898)	(915,010)
Available-for-sale securities	786	996
Total accumulated other comprehensive loss	(828,304)	(910,598)
Pre-tax activity in OCI related to derivatives
Beginning of period unrealized gain in accumulated OCI	5,664	49,407	65,582
Net (decrease) increase in fair value of derivatives	(16,508)	(24,424)	26,491
Net gains reclassified from OCI, offsetting the price paid to suppliers	7,314	(19,319)	(42,666)
End of period unrealized gain (loss) in accumulated OCI	$ (3,530)	$ 5,664	$ 49,407