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Pension and Other Postretirement Benefits (Assumptions and Sensativity) (Details)
12 Months Ended
Apr. 30, 2013
Apr. 30, 2012
Apr. 30, 2011
Pension Benefits [Member]
     
Assumptions used in computing benefit plan obligations      
Discount rate (percent) 4.08% 4.92%  
Rate of salary increase (percent) 4.00% 4.00%  
Assumptions used in computing benefit plan expense      
Discount rate (percent) 4.92% 5.67% 5.91%
Rate of salary increase (percent) 4.00% 4.00% 4.00%
Expected return on plan assets (percent) 7.75% 8.25% 8.50%
Medical and Life Insurance Benefits [Member]
     
Assumptions used in computing benefit plan obligations      
Discount rate (percent) 4.36% 4.84%  
Assumptions used in computing benefit plan expense      
Discount rate (percent) 4.84% 5.59% 5.78%
Present rate before age 65 [Member] | Medical and Life Insurance Benefits [Member]
     
Assumed health care cost trend rates      
Present rate (percent) 8.00% 8.00%  
Present rate age 65 and after [Member] | Medical and Life Insurance Benefits [Member]
     
Assumed health care cost trend rates      
Present rate (percent) 7.00% 8.00%