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Debt and Financing Arrangements (Details) (USD $)
12 Months Ended 12 Months Ended 12 Months Ended
Apr. 30, 2012
Apr. 30, 2011
Apr. 30, 2010
Apr. 30, 2009
Apr. 30, 2011
7.94% Series C Senior Notes due September 1, 2010 [Member]
Sep. 01, 2010
7.94% Series C Senior Notes due September 1, 2010 [Member]
Apr. 30, 2010
7.94% Series C Senior Notes due September 1, 2010 [Member]
Apr. 30, 2011
4.78% Senior Notes due June 1, 2014 [Member]
Apr. 30, 2010
4.78% Senior Notes due June 1, 2014 [Member]
Apr. 30, 2011
6.12% Senior Notes due November 1, 2015 [Member]
Apr. 30, 2010
6.12% Senior Notes due November 1, 2015 [Member]
Apr. 30, 2011
6.63% Senior Notes due November 1, 2018 [Member]
Apr. 30, 2010
6.63% Senior Notes due November 1, 2018 [Member]
Apr. 01, 2013
5.55% Senior Notes due April 1, 2022 [Member]
Apr. 30, 2011
5.55% Senior Notes due April 1, 2022 [Member]
Apr. 30, 2010
5.55% Senior Notes due April 1, 2022 [Member]
Jun. 01, 2020
4.50% Senior Notes due June 1, 2025 [Member]
Apr. 30, 2011
4.50% Senior Notes due June 1, 2025 [Member]
Jun. 15, 2010
4.50% Senior Notes due June 1, 2025 [Member]
Apr. 30, 2010
4.50% Senior Notes due June 1, 2025 [Member]
Apr. 30, 2011
Unsecured Senior Notes [Member]
Jan. 31, 2011
Group of six lenders [Member]
Jan. 31, 2011
Amounts Maturing during the Year [Member]
Apr. 30, 2011
Interest rate contract [Member]
Apr. 30, 2010
Interest rate contract [Member]
Long-term debt                                                  
Company issued Senior Notes         $ 0   $ 10,000,000 $ 100,000,000 $ 100,000,000 $ 24,000,000 $ 24,000,000 $ 380,039,000 $ 376,000,000   $ 400,000,000 $ 400,000,000   $ 400,000,000   $ 0          
Total long-term debt   1,304,039,000 910,000,000                                            
Current portion of long-term debt   0 10,000,000                                            
Total long-term debt less current portion   1,304,039,000 900,000,000                                            
Debt and Financing Arrangements (Textuals)                                                  
Percentage of senior notes   6.60%       7.94%   4.78%   6.12%   6.63%     5.55%       4.50%            
Issued 4.50 percent senior notes with a final maturity on June1, 2025                                     400,000,000            
First period payment of Senior Notes                           50,000,000     100,000,000                
Average maturity of Senior Notes                                         12 years        
Payment of Senior Notes   10,000,000 275,000,000 0 10,000,000                                        
Outstanding stand By Letter of credit   7,100,000                                              
Revolving credit facility for general corporate purposes utilized 240,000,000                                                
Interest Paid   62,075,000 76,461,000 52,918,000                                          
Line of Credit Facility (Textuals) [Line Items]                                                  
Unsecured revolving credit facility with banks                                           600,000,000 180,000,000    
Notional amount                                               376,000,000 0
Interest rate swap                                               $ 4,000,000