Earnings per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
	Jul. 31, 2012	Jul. 31, 2011
Computation of net income per share
Net income	$ 110,863	$ 111,523
Net income allocated to participating securities	954	1,127
Net income allocated to common stockholders	109,909	110,396
Weighted-average common shares outstanding	109,418,592	113,122,789
Net income per common share	$ 1.00	$ 0.98
Computation of net income per share - assuming dilution
Net income	110,863	111,523
Net income allocated to participating securities	954	1,127
Net income allocated to common stockholders	$ 109,909	$ 110,396
Weighted-average common shares outstanding	109,418,592	113,122,789
Dilutive effect of stock options	29,536	57,637
Weighted-average common shares outstanding - assuming dilution	109,448,128	113,180,426
Net income per common share - assuming dilution	$ 1.00	$ 0.98
Weighted-average common shares, basic and diluted
Weighted-average common shares outstanding	109,418,592	113,122,789
Weighted-average participating shares outstanding	949,679	1,154,758
Total weighted-average shares outstanding	110,368,271	114,277,547
Dilutive effect of stock options	29,536	57,637
Total weighted-average shares outstanding - assuming dilution	110,397,807	114,335,184