Convertible Notes Payable (Details) (USD $) In Thousands, unless otherwise specified	Apr. 30, 2013
Carrying value and estimated current fair value of outstanding convertible notes
Carrying value	$ 1,209,814
Fair Value	1,272,322	[1]
4.0% Convertible Senior Notes due March 15, 2015 [Member]
Carrying value and estimated current fair value of outstanding convertible notes
Carrying value	187,818	[2]
Fair Value	204,609	[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	463,750	[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	387,625	[1]
Interest rate on convertible notes	3.75%
4.0% Convertible Senior Notes due December 15, 2020 [Member]
Carrying value and estimated current fair value of outstanding convertible notes
Carrying value	171,996	[3]
Fair Value	$ 216,338	[1],[3]
Interest rate on convertible notes	4.00%

[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

[3] Includes unamortized discount and accretion of principal.