Earnings per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Jan. 31, 2013	Jan. 31, 2012	Jan. 31, 2013	Jan. 31, 2012
Computation of net income per common share:
Net income	$ 154,168	$ 116,844	$ 413,878	$ 355,614
Net income allocated to participating securities	1,341	974	3,594	3,394
Net income allocated to common stockholders	152,827	115,870	410,284	352,220
Weighted-average common shares outstanding	107,528,722	112,493,822	108,405,604	112,783,014
Net income per common share	$ 1.42	$ 1.03	$ 3.78	$ 3.12
Computation of net income per common share - assuming dilution:
Net income	154,168	116,844	413,878	355,614
Net income allocated to participating securities	1,341	973	3,593	3,394
Net income allocated to common stockholders	$ 152,827	$ 115,871	$ 410,285	$ 352,220
Weighted-average common shares outstanding	107,528,722	112,493,822	108,405,604	112,783,014
Dilutive effect of stock options	19,655	49,125	25,263	52,811
Weighted-average common shares outstanding - assuming dilution	107,548,377	112,542,947	108,430,867	112,835,825
Net income per common share - assuming dilution	$ 1.42	$ 1.03	$ 3.78	$ 3.12
Weighted-average common shares, basic and diluted
Weighted-average common shares outstanding	107,528,722	112,493,822	108,405,604	112,783,014
Weighted-average participating shares outstanding	943,545	945,330	949,527	1,086,897
Total weighted-average shares outstanding	108,472,267	113,439,152	109,355,131	113,869,911
Dilutive effect of stock options	19,655	49,125	25,263	52,811
Total weighted-average shares outstanding - assuming dilution	108,491,922	113,488,277	109,380,394	113,922,722