Senior and Subordinated Debt (Details) (USD $) In Thousands, unless otherwise specified	Apr. 30, 2012	Jan. 31, 2012
Senior Notes
Total Senior Notes	$ 1,009,610	$ 1,009,529
Subordinated Debt
Senior and Subordinated debt	1,038,610	1,038,529
3.625% Puttable Equity-Linked Senior Notes due 2014 [Member]
Senior Notes
Total Senior Notes	199,213	199,132
7.625% Senior Notes due 2015 [Member]
Senior Notes
Total Senior Notes	178,253	178,253
5.000% Convertible Senior Notes due 2016 [Member]
Senior Notes
Total Senior Notes	50,000	50,000
6.500% Senior Notes due 2017 [Member]
Senior Notes
Total Senior Notes	132,144	132,144
4.250% Convertible Senior Notes due 2018 [Member]
Senior Notes
Total Senior Notes	350,000	350,000
7.375% Senior Notes due 2034 [Member]
Senior Notes
Total Senior Notes	100,000	100,000
Subordinate Tax Revenue Bonds due 2013 [Member]
Subordinated Debt
Subordinate Tax Revenue Bonds due 2013	$ 29,000	$ 29,000