Debt (Convertible Senior Debentures) (Narrative) (Details) (USD $)			3 Months Ended		9 Months Ended
	Oct. 31, 2011	Sep. 30, 2011	Oct. 31, 2011 Convertible Senior Debentures [Member]	Oct. 31, 2010 Convertible Senior Debentures [Member]	Oct. 31, 2011 Convertible Senior Debentures [Member]	Oct. 31, 2010 Convertible Senior Debentures [Member]	Jan. 31, 2011 Convertible Senior Debentures [Member]	Dec. 31, 2006 Convertible Senior Debentures [Member]
Debt Instrument [Line Items]
Convertible senior debentures, issuance date					December 2006
Convertible senior debentures, face value			$ 350,000,000		$ 350,000,000		$ 350,000,000	$ 350,000,000
Debt, maturity date					Dec. 01, 2026
Convertible senior debentures, annual interest rate			2.75%		2.75%			2.75%
Borrowing rate of non-convertible debt					6.30%
Interest expense			2,400,000	2,400,000	7,200,000	7,200,000
Non-cash interest expense			2,500,000	2,500,000	7,500,000	7,500,000
Unamortized debt discount	1,300,000		1,285,000		1,285,000		8,993,000
Line of Credit Facility, Maximum Borrowing Capacity	$ 1,400,000,000	$ 750,000,000