Debt and Financing Arrangements (Details) (USD $)	3 Months Ended	12 Months Ended							3 Months Ended							3 Months Ended	9 Months Ended
	Oct. 31, 2011	Apr. 30, 2011	Jul. 29, 2016 Bank	Jan. 31, 2012	Oct. 18, 2011	Jan. 31, 2011	Jan. 31, 2012 Interest rate contracts [Member]	Apr. 30, 2011 Interest rate contracts [Member]	Oct. 31, 2011 Cash Flow Hedging [Member] Interest rate contracts [Member]	Jan. 31, 2012 4.78% Senior Notes due June 1, 2014 [Member]	Apr. 30, 2011 4.78% Senior Notes due June 1, 2014 [Member]	Jan. 31, 2012 6.12% Senior Notes due November 1, 2015 [Member]	Apr. 30, 2011 6.12% Senior Notes due November 1, 2015 [Member]	Jan. 31, 2012 6.63% Senior Notes due November 1, 2018 [Member]	Apr. 30, 2011 6.63% Senior Notes due November 1, 2018 [Member]	Oct. 31, 2011 3.50% Notes due October 15, 2021 [Member]	Jan. 31, 2012 3.50% Notes due October 15, 2021 [Member]	Oct. 18, 2011 3.50% Notes due October 15, 2021 [Member]	Apr. 30, 2011 3.50% Notes due October 15, 2021 [Member]	Jan. 31, 2012 5.55% Senior Notes due April 1, 2022 [Member]	Apr. 30, 2011 5.55% Senior Notes due April 1, 2022 [Member]	Jan. 31, 2012 4.50% Senior Notes due June 1, 2025 [Member]	Apr. 30, 2011 4.50% Senior Notes due June 1, 2025 [Member]
Long-term debt
Company issued Senior Notes										$ 100,000,000	$ 100,000,000	$ 24,000,000	$ 24,000,000	$ 398,601,000	$ 380,039,000					$ 400,000,000	$ 400,000,000	$ 400,000,000	$ 400,000,000
Notes Payable, Noncurrent																	748,601,000		0
Total long-term debt		1,304,039,000		2,071,202,000
Debt and Financing Arrangements (Textual) [Abstract]
Percentage of Notes					3.50%					4.78%	4.78%	6.12%	6.12%	6.63%	6.63%		3.50%		3.50%	5.55%	5.55%	4.50%	4.50%
First period payment of Senior Notes																				50,000,000		100,000,000
Notional amount							0	376,000,000
Fair value adjustment of the interest rate swap		4,000,000
Unamortized benefit				22,600,000
Cash received on early termination of interest rate swap	27,000,000
Interest receivable on early termination of interest rate swap	3,100,000
Gain on early termination agreement	23,900,000
Revolving Credit Facility Maximum Borrowing Capacity			1,000,000,000			600,000,000
Notes including Unamortized Discount																		750,000,000
Unamortized discount																		1,400,000
Effective rate of notes																	3.52%
Net proceeds																748,600,000
Notional amount of cash flow hedge instruments	500,000,000
Termination loss on interest rate swap									$ 6,200,000
Number of banks			10