Bank Revolving Credit Facility (Details Textuals) (USD $)	4 Months Ended	6 Months Ended			6 Months Ended		6 Months Ended			6 Months Ended
	Sep. 07, 2011	Jul. 31, 2011	Mar. 30, 2011	Jan. 31, 2011	Jul. 31, 2011 3.625% Puttable Equity-Linked Senior Notes due 2011 [Member]	Apr. 21, 2011 Original Credit Facility [Member]	Jul. 31, 2011 Amended Credit Facility [Member]	Jul. 13, 2011 Amended Credit Facility [Member]	Apr. 21, 2011 Amended Credit Facility [Member]	Jul. 31, 2011 4.25% Convertible Senior Notes due 2018 [Member]	Jul. 13, 2011 4.25% Convertible Senior Notes due 2018 [Member]
Bank Revolving Credit Facility (Textuals)
Number of members in bank group			13
Total borrowings of credit agreement		$ 450,000,000		$ 470,336,000		$ 425,000,000			$ 450,000,000
Credit agreement maturity date							March 30, 2014 and one 12-month extension option
Interest rate of borrowings							LIBOR, subject to a floor of 100 basis points, plus 3.75%
Amount of available borrowing that can be used for letters of credit or surety bonds							100,000,000
Limitation of common stock dividends or common stock repurchases in any four fiscal quarter period								24,000,000
Convertible notes authorized to issue as per amendment											350,000,000
Interest Rate of Debt Instrument											4.25%
Percentage of Net proceeds from issuance of Senior note to be used to retire Subject debt										75.00%
Net proceeds from issuance of Senior note to be used to retire Subject debt										254,531,000
Maximum number of days Subsequent to closing of issuance of notes to retire subject debt										180 days
Extension available to retire debt										90 days
Remaining debt to be retired		73,113,000			46,891,000
Additional Bank Revolving Credit Facility (Textuals) [Abstract]
Proceeds from payments for deposits applied to debt retirements	$ 181,418,000	$ 133,118,000