Convertible Notes Payable (Details) (USD $) In Thousands, unless otherwise specified	Jul. 31, 2012
Carrying value and estimated current fair value of outstanding convertible notes
Carrying value	$ 1,442,108
Fair Value	1,455,695	[1]
0.25% Convertible Senior Notes due May 1, 2013 [Member]
Carrying value and estimated current fair value of outstanding convertible notes
Carrying value	216,210
Fair Value	212,967	[1]
Interest rate on convertible notes	0.25%
4.0% Convertible Senior Notes due March 15, 2015 [Member]
Carrying value and estimated current fair value of outstanding convertible notes
Carrying value	375,898	[2]
Fair Value	422,415	[1],[2]
Interest rate on convertible notes	4.00%
0.875% Convertible Senior Notes due June 15, 2017 [Member]
Carrying value and estimated current fair value of outstanding convertible notes
Carrying value	500,000
Fair Value	428,750	[1]
Interest rate on convertible notes	0.875%
3.75% Convertible Senior Notes due October 15, 2018 [Member]
Carrying value and estimated current fair value of outstanding convertible notes
Carrying value	350,000
Fair Value	$ 391,563	[1]
Interest rate on convertible notes	3.75%

[1] The fair value reported above is based on the quoted prices for the notes on the date above.

[2] Includes unamortized bond premium related to embedded redemption feature