Debt and Financing Arrangements (Details) (USD $)	3 Months Ended		12 Months Ended
	Oct. 31, 2011	Jul. 31, 2011	Apr. 30, 2011	Aug. 31, 2011	Jul. 29, 2011	Jan. 31, 2011	Jul. 31, 2011 4.78% Senior Notes due June 1, 2014 [Member]	Apr. 30, 2011 4.78% Senior Notes due June 1, 2014 [Member]	Jul. 31, 2011 6.12% Senior Notes due November 1, 2015 [Member]	Apr. 30, 2011 6.12% Senior Notes due November 1, 2015 [Member]	Jul. 31, 2011 6.63% Senior Notes due November 1, 2018 [Member]	Apr. 30, 2011 6.63% Senior Notes due November 1, 2018 [Member]	Apr. 01, 2013 5.55% Senior Notes due April 1, 2022 [Member]	Jul. 31, 2011 5.55% Senior Notes due April 1, 2022 [Member]	Apr. 30, 2011 5.55% Senior Notes due April 1, 2022 [Member]	Jun. 01, 2020 4.50% Senior Notes due June 1, 2025 [Member]	Jul. 31, 2011 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	$ 394,489,000	$ 380,039,000		$ 400,000,000	$ 400,000,000		$ 400,000,000	$ 400,000,000
Total long-term debt		1,318,489,000	1,304,039,000
Debt and Financing Arrangements (Textuals)
Percentage of senior notes							4.78%	4.78%	6.12%	6.12%	6.63%	6.63%		5.55%	5.55%		4.50%	4.50%
First period payment of Senior Notes													50,000,000			100,000,000
Notional amount		376,000,000
Interest rate swap		18,500,000	4,000,000
Cash received on early termination of interest rate swap	27,000,000
Gain on early termination agreement	24,200,000
Revolving Credit Facility Maximum Borrowing Capacity					1,000,000,000	600,000,000
Revolving credit facility at weighted average interest rate		1.52%		1.42%
Revolving Credit Facility Outstanding Balance		$ 306,700,000	$ 0	$ 480,000,000
Number of banks					10