Bank Revolving Credit Facility (Details Textual) (USD $)					9 Months Ended			9 Months Ended
	Oct. 31, 2011	Mar. 30, 2011 Entity	Jan. 31, 2011	Apr. 21, 2011 Original Credit Facility [Member]	Oct. 31, 2011 Amended Credit Facility [Member]	Jul. 13, 2011 Amended Credit Facility [Member]	Apr. 21, 2011 Amended Credit Facility [Member]	Oct. 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 (Textual)
Convertible notes authorized to issue as per amendment									$ 350,000,000
Percentage of Net proceeds from issuance of Senior note to be used to retire debt								75.00%
Net proceeds from issuance of Senior note to be used to retire debt								254,531,000
Maximum number of days subsequent to closing of issuance of notes to retire debt								180 days
Extension available to retire debt								90 days
Line of Credit Facility [Line Items]
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%
Debt instrument basis spread on variable rate					3.75%
Debt instrument floor rate					1.00%
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 consecutive fiscal quarter period						$ 24,000,000
Additional Bank Revolving Credit Facility (Textual) [Abstract]
Number of members in bank group		13