Earnings per Share (Details) (USD $) In Thousands, except Share data	3 Months Ended
	Jul. 31, 2011	Jul. 31, 2010
Computation of net income per share:
Net income	$ 111,523	$ 102,881
Net income allocated to participating securities	1,127	987
Net income allocated to common shareholders	110,396	101,894
Weighted-average common shares outstanding	113,122,789	118,156,815
Net income per Common Share, in USD per share	$ 0.98	$ 0.86
Computation of net income per share - assuming dilution:
Net income	111,523	102,881
Net income allocated to participating securities	1,127	986
Net income allocated to common shareholders	$ 110,396	$ 101,895
Weighted-average common shares outstanding	113,122,789	118,156,815
Dilutive effect of stock options, in shares	57,637	139,564
Weighted-average common shares outstanding - assuming dilution	113,180,426	118,296,379
Net income per common share - assuming dilution, in USD per share	$ 0.98	$ 0.86
Weighted-average common shares, basic and diluted
Weighted-average common shares outstanding	113,122,789	118,156,815
Weighted Average Participating Shares Outstanding	1,154,758	1,144,111
Total Weighted-average shares outstanding	114,277,547	119,300,926
Dilutive effect of stock options, in shares	57,637	139,564
Total Weighted-average shares outstanding - assuming dilution	114,335,184	119,440,490