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Supplementary Quarterly Financial Information (Details) - USD ($)
$ / shares in Units, $ in Thousands
 3 Months Ended 12 Months Ended
Apr. 30, 2018
Jan. 31, 2018
Oct. 31, 2017
Jul. 31, 2017
Apr. 30, 2017
Jan. 31, 2017
Oct. 31, 2016
Jul. 31, 2016
Apr. 30, 2018
Apr. 30, 2017
Apr. 30, 2016
Supplementary Quarterly Financial Information - Results By Quarter (Unaudited) [Abstract]                      
Revenue $ 477,300 $ 455,700 $ 451,700 $ 411,400 $ 452,200 $ 436,400 $ 425,600 $ 404,300 $ 1,796,103 $ 1,718,530 $ 1,727,037
Gross profit 351,800 330,500 331,900 296,700 332,900 320,100 314,000 290,800 1,310,900 1,257,800  
Operating income 74,800 67,400 82,800 14,500 63,500 51,200 47,700 43,800 239,535 206,153 188,113
Net income $ 54,200 $ 68,800 $ 60,000 $ 9,200 $ 46,700 $ 47,400 $ (11,500) $ 31,000 $ 192,186 $ 113,643 $ 145,782
Earnings Per Share [Abstract]                      
Basic (in dollars per share) $ 0.95 [1] $ 1.21 [1] $ 1.06 [1] $ 0.16 [1] $ 0.82 [1] $ 0.83 [1] $ (0.20) [1] $ 0.54 [1] $ 3.37 $ 1.98 $ 2.51
Diluted (in dollars per share) $ 0.93 [1] $ 1.19 [1] $ 1.04 [1] $ 0.16 [1] $ 0.81 [1] $ 0.82 [1] $ (0.20) [1] $ 0.53 [1] $ 3.32 $ 1.95 $ 2.48
[1] The sum of the quarterly earnings per share amounts may not agree to the respective annual amounts due to rounding.