Senior and Subordinated Debt (Details) (USD $)	Jul. 31, 2013	Jan. 31, 2013	Jul. 31, 2012
Senior Notes
Total Senior Notes	$ 1,052,511,000	$ 1,003,969,000
Subordinated Debt [Abstract]
Senior and Subordinated debt	1,052,511,000	1,032,969,000
Puttable Equity-Linked Senior Notes due 2014 [Member]
Debt Instrument [Line Items]
Interest rate	3.625%	3.625%
Senior Notes
Total Senior Notes	1,112,000	199,457,000
Senior Notes due 2015 [Member]
Debt Instrument [Line Items]
Interest rate	7.625%	7.625%
Senior Notes
Total Senior Notes	0	53,253,000
Convertible Senior Notes due 2016 [Member]
Debt Instrument [Line Items]
Interest rate	5.00%	5.00%
Senior Notes
Total Senior Notes	50,000,000	50,000,000
Senior Notes due 2017 [Member]
Debt Instrument [Line Items]
Interest rate	6.50%	6.50%
Senior Notes
Total Senior Notes	132,144,000	132,144,000
Convertible Senior Notes due 2018 [Member]
Debt Instrument [Line Items]
Interest rate	4.25%	4.25%
Senior Notes
Total Senior Notes	350,000,000	350,000,000
Convertible Senior Notes due 2020 [Member]
Debt Instrument [Line Items]
Interest rate	3.625%	3.625%
Senior Notes
Total Senior Notes	300,000,000	0
Senior Notes due 2034 [Member]
Debt Instrument [Line Items]
Interest rate	7.375%	7.375%	7.375%
Senior Notes
Total Senior Notes	219,255,000	219,115,000
Subordinate Tax Revenue Bonds due 2013 [Member]
Subordinated Debt [Abstract]
Subordinate Tax Revenue Bonds due 2013	$ 0	$ 29,000,000