Long-Term Debt - Additional Information (Details) (USD $)	12 Months Ended						1 Months Ended	12 Months Ended	1 Months Ended												0 Months Ended	1 Months Ended	12 Months Ended	0 Months Ended
	May 31, 2013 payment	May 31, 2012	May 31, 2011	May 31, 2013 Guarantee Of Joint Venture Debt	Aug. 25, 2011 Maximum Increase	Aug. 25, 2011 Minimum Increase	Aug. 25, 2011 Senior Secured Revolving Credit Facility	May 31, 2013 Senior Secured Revolving Credit Facility	Aug. 25, 2011 Senior Secured Revolving Credit Facility Minimum	Aug. 25, 2011 Senior Secured Revolving Credit Facility Maximum	Aug. 25, 2011 Sub-Limit For Letters Of Credit	Aug. 25, 2011 Sub-Limit Swing Line Loans	Aug. 25, 2011 Libor Rate Plus Minimum	Aug. 25, 2011 Libor Rate Plus Maximum	Aug. 25, 2011 Base Rate Plus Minimum	Aug. 25, 2011 Base Rate Plus Maximum	Aug. 25, 2011 Federal Funds Rate Plus	Aug. 25, 2011 One-Month Libor Rate Plus	May 31, 2013 Credit Agreement Minimum	May 31, 2013 Credit Agreement Maximum	Aug. 10, 2010 9.25% Senior Notes	Aug. 10, 2010 9.25% Senior Notes	May 31, 2013 9.25% Senior Notes	Jul. 27, 2010 7.25% Senior Notes Due 2013	Aug. 10, 2010 7.25% Senior Notes Due 2013
Debt Instrument [Line Items]
Maximum amount available to borrow under agreement							$ 200,000,000				$ 50,000,000	$ 15,000,000
Credit facility expiration date								August 25, 2016
Line of credit facility, interest rate description							Amounts drawn under the credit facility bear annual interest either at the LIBOR rate plus a margin of 2.0% to 2.75% or at a base rate plus a margin of 1.0% to 1.75%. The base rate is the higher of the federal funds rate plus 0.5%, the prime rate established by Bank of America, N.A. or the one-month LIBOR rate plus 1.0%. The interest rate margins are determined based on the Company’s fixed charge coverage ratio
Variable interest rate													2.00%	2.75%	1.00%	1.75%	0.50%	1.00%
Commitment fee calculated on unused portion per year									0.375%	0.50%
Borrowing base agreement amount	136,900,000				20,000,000	10,700,000.0
Unused portion of borrowing amount	25,000,000
Fixed charge coverage ratio, maximum allowed according to covenant	1.0
Fixed charge coverage ratio	0.40							1.0											1.0	1.0
Borrowing limited to an amount under credit facility	111,900,000
Borrowings outstanding								0
Borrowing base used to support letters of credit	32,600,000
Line of credit facility, remaining borrowing capacity	79,300,000
Future foreign subsidiaries equity interest available for credit facility guarantees	66.00%
Line of credit facility, covenant terms	The credit agreement contains a number of covenants restricting, among other things, prepayment or redemption of our senior notes, distributions and dividends on and repurchases of our capital stock, acquisitions and investments, indebtedness, liens and affiliate transactions. We are permitted to pay cash dividends on our common stock as long as the credit facility is not in default, the fixed charge coverage ratio is greater than 1.0 to 1.0 and borrowing availability under the borrowing base is more than $40 million. When our fixed charge coverage ratio is less than 1.0 to 1.0, we are permitted to pay cash dividends on our common stock not to exceed $2.5 million in any single instance (which shall not occur more than four times in any calendar year) or $10 million in the aggregate during any calendar year as long as the credit facility is not in default and borrowing availability is more than the greater of $60 million or 30% of the aggregate commitments of all lenders.
Borrowing availability under credit agreement	40,000,000
Cash dividend on common stock	2,500,000.0
Number of cash dividend payments allowed annually, maximum	4
Permitted to pay cash dividends on common stock aggregate during any calendar year	10,000,000
Minimum borrowing availability required to pay cash dividends	60,000,000
Line of credit facility percentage of aggregate commitments to lenders	30.00%
Senior note aggregate principal amount																					650,000,000	650,000,000	650,000,000
Senior notes offering price in percentage																						100.00%
Senior notes, interest rate																					9.25%	9.25%	9.25%	7.25%	7.25%
Cash tender offer for outstanding aggregate principal amount																								550,000,000
Principal amount of notes purchased pursuant to tender offer and consent solicitation																								536,600,000
Amount paid for purchase price and consent fees																								547,700,000
Extinguishment of debt																								13,400,000
Gains (losses) on extinguishment of debt	0	0	(29,619,000)																					(29,600,000)
Consent fees																								11,400,000
Unamortized debt discount																								18,200,000
Senior notes maturity date																					2020		2020
Redemption of aggregate principal amount of notes, percentage	35.00%
Percentage of principal amount redemption price	109.25%																							101.813%
Required purchase price of principal amount, percentage	101.00%
Principal payments due on long-term debt, year one	1,800,000
Principal payments due on long-term debt, year two	1,800,000
Principal payments due on long-term debt, year three	1,900,000
Principal payments due on long-term debt, year four	1,700,000
Principal payments due on long-term debt, year five	300,000
Interest incurred on long-term debt	69,300,000	68,500,000	66,300,000
Interest amount capitalized	36,500,000	33,700,000	18,700,000
Total amount of interest paid	66,400,000	66,300,000	60,500,000
Percentage of joint venture's debt guaranteed	50.00%			50.00%
Debt guaranteed under joint venture's	$ 657,505,000	$ 655,778,000		$ 10,700,000