Long-Term Debt - Additional Information (Detail) (USD $)	6 Months Ended		6 Months Ended	12 Months Ended	1 Months Ended		1 Months Ended								1 Months Ended			1 Months Ended			6 Months Ended	88 Months Ended		1 Months Ended	6 Months Ended	66 Months Ended		1 Months Ended
	Sep. 30, 2013	Mar. 31, 2013	Sep. 30, 2013 Industrial Revenue Bonds [Member]	Dec. 31, 1998 Industrial Revenue Bonds [Member]	Sep. 26, 2012 Letter Of Credit [Member]	Sep. 30, 2013 Letter Of Credit [Member]	Sep. 26, 2012 Credit Facility [Member]	Dec. 16, 2010 Credit Facility [Member]	Sep. 30, 2013 Credit Facility [Member]	Mar. 31, 2013 Credit Facility [Member]	Sep. 26, 2012 Credit Facility [Member] Revolving Credit Facility [Member]	Dec. 16, 2010 Credit Facility [Member] Revolving Credit Facility [Member]	Sep. 26, 2012 Credit Facility [Member] Sub Limit Swing Line Loans [Member]	Dec. 16, 2010 Credit Facility [Member] Sub Limit Swing Line Loans [Member]	Sep. 26, 2012 Credit Facility [Member] Minimum [Member]	Sep. 26, 2012 Credit Facility [Member] Maximum [Member]	Sep. 26, 2012 Credit Facility [Member] Federal Funds Rate [Member] Minimum [Member]	Sep. 26, 2012 Credit Facility [Member] Federal Funds Rate [Member] Maximum [Member]	Sep. 26, 2012 Credit Facility [Member] LIBOR [Member] Minimum [Member]	Sep. 26, 2012 Credit Facility [Member] LIBOR [Member] Maximum [Member]	Sep. 30, 2013 2005 Note Purchase Agreement [Member]	Mar. 31, 2013 2005 Note Purchase Agreement [Member]	Nov. 15, 2005 2005 Note Purchase Agreement [Member] Loan	Nov. 30, 2012 2005 Note Purchase Agreement [Member] Series 2005A Tranche A [Member]	Sep. 30, 2013 2007 Note Purchase Agreement [Member]	Mar. 31, 2013 2007 Note Purchase Agreement [Member]	Oct. 02, 2007 2007 Note Purchase Agreement [Member] Loan	Aug. 31, 2011 Shelf Agreement [Member]
Debt Instrument [Line Items]
Credit facility, principal balance											$ 400,000,000	$ 300,000,000	$ 25,000,000	$ 15,000,000
Credit facility, maturity date								Dec. 16, 2015
Credit facility, interest rate description							At the option of the Company, outstanding principal amounts on the Credit Facility bear interest at a variable rate equal to (i) LIBOR, plus an agreed margin (ranging from 100 to 225 basis points), which is to be established quarterly based upon the Company’s ratio of consolidated EBITDA, defined as earnings before interest, taxes, depreciation and amortization, to the Company’s consolidated indebtedness (the “Leverage Ratio”), or (ii) an alternative base rate which is the higher of (a) the prime rate or (b) the federal funds rate plus 1/2%, per annum plus an agreed margin (ranging from 0 to 125 basis points)
Variable margin																	0.00%	1.25%	1.00%	2.25%
Federal funds rate plus																		0.50%
Consolidated funded indebtedness ratio							3.5:1.0 or less and an interest coverage ratio
Interest coverage ratio							2.5
Restricted payments maximum amount allowed for cash dividend							50,000,000
Unused line of credit commitment fee based on leverage ratio															0.10%	0.35%
Borrowings outstanding under Credit Facility	263,000,000
Bank credit facility, borrowings available	129,600,000
Restricted payments maximum consolidated funded indebtedness ratio							3
Letter of credit facility						50,000,000
Bank credit facility, one-time fee					0.125%
Letters of credit outstanding, amount	7,400,000					7,400,000
Senior notes, sale																							200,000,000				200,000,000
Number of tranches																							3				4
Senior notes, repurchased amount																						81,100,000				122,000,000
Retirement of senior note																								4,700,000
Senior notes, payment terms																					Interest for each tranche of Notes is payable semi-annually on the 15thday of May and the 15thday of November of each year until all principal is paid for the respective tranche				Interest for each tranche of Notes is payable semi-annually on the second day of April and the second day of October of each year until all principal is paid for the respective tranche
Senior notes, permitted minimum aggregate principal amount prepayment without penalty	10.00%
Percentage of face value to be paid if notes are prepaid	100.00%
Senior notes, calculation of make-whole amount, description	Discounting the remaining scheduled payments of interest and principal of the Senior Notes being prepaid at a discount rate equal to the sum of 50 basis points
Discount on senior notes principal and interest	0.50%
Proceeds from issuance of tax-exempt and taxable industrial revenue bonds				150,000,000
Industrial revenue bonds amount placed in escrow account			47,000,000
Industrial revenue bonds outstanding	455,259,000	489,259,000	103,000,000						263,000,000	297,000,000
Remaining amount of industrial revenue bonds transferred in acquisition			103,000,000
Senior unsecured notes																												$ 75,000,000
Repayment term, years																												Two