Earnings per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Oct. 31, 2012	Oct. 31, 2011	Oct. 31, 2012	Oct. 31, 2011
Computation of net income per common share:
Net income	$ 148,847	$ 127,247	$ 259,710	$ 238,770
Net income allocated to participating securities	1,299	1,297	2,253	2,424
Net income allocated to common stockholders	147,548	125,950	257,457	236,346
Weighted-average common shares outstanding	108,269,499	112,731,758	108,844,046	112,927,273
Net income per common share	$ 1.36	$ 1.12	$ 2.37	$ 2.09
Computation of net income per common share - assuming dilution:
Net income	148,847	127,247	259,710	238,770
Net income allocated to participating securities	1,299	1,297	2,253	2,423
Net income allocated to common stockholders	$ 147,548	$ 125,950	$ 257,457	$ 236,347
Weighted-average common shares outstanding	108,269,499	112,731,758	108,844,046	112,927,273
Dilutive effect of stock options	26,600	51,670	28,068	54,654
Weighted-average common shares outstanding - assuming dilution	108,296,099	112,783,428	108,872,114	112,981,927
Net income per common share - assuming dilution	$ 1.36	$ 1.12	$ 2.36	$ 2.09
Weighted-average common shares, basic and diluted
Weighted-average common shares outstanding	108,269,499	112,731,758	108,844,046	112,927,273
Weighted-average participating shares outstanding	955,356	1,161,277	952,518	1,158,018
Total weighted-average shares outstanding	109,224,855	113,893,035	109,796,564	114,085,291
Dilutive effect of stock options	26,600	51,670	28,068	54,654
Total weighted-average shares outstanding - assuming dilution	109,251,455	113,944,705	109,824,632	114,139,945