Earnings per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Oct. 31, 2011	Oct. 31, 2010	Oct. 31, 2011	Oct. 31, 2010
Computation of net income per share:
Net income	$ 127,247	$ 149,726	$ 238,770	$ 252,607
Net income allocated to participating securities	1,297	1,501	2,424	2,478
Net income allocated to common shareholders	125,950	148,225	236,346	250,129
Weighted-average common shares outstanding	112,731,758	118,313,698	112,927,273	118,235,258
Net income per common share	$ 1.12	$ 1.25	$ 2.09	$ 2.12
Computation of net income per share - assuming dilution:
Net income	127,247	149,726	238,770	252,607
Net income allocated to participating securities	1,297	1,500	2,423	2,476
Net income allocated to common shareholders	$ 125,950	$ 148,226	$ 236,347	$ 250,131
Weighted-average common shares outstanding	112,731,758	118,313,698	112,927,273	118,235,258
Dilutive effect of stock options	51,670	130,397	54,654	134,980
Weighted-average common shares outstanding - assuming dilution	112,783,428	118,444,095	112,981,927	118,370,238
Net income per common share - assuming dilution	$ 1.12	$ 1.25	$ 2.09	$ 2.11
Weighted-average common shares, basic and diluted
Weighted-average common shares outstanding	112,731,758	118,313,698	112,927,273	118,235,258
Weighted-average participating shares outstanding	1,161,277	1,198,303	1,158,018	1,171,207
Total weighted-average shares outstanding	113,893,035	119,512,001	114,085,291	119,406,465
Dilutive effect of stock options	51,670	130,397	54,654	134,980
Total Weighted-average shares outstanding - assuming dilution	113,944,705	119,642,398	114,139,945	119,541,445