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Segment Information and Sales to Significant Customers - Revenue and Customer Information (Detail) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2016
Revenue from External Customer [Line Items]                      
Total revenue $ 1,002,588 $ 1,002,198 $ 992,340 $ 977,711 $ 979,724 $ 966,695 $ 966,009 $ 954,727 $ 3,974,837 $ 3,867,155 $ 3,718,229
AmdocsONE [Member]                      
Revenue from External Customer [Line Items]                      
Total revenue                 3,930,186 3,809,088 3,629,698
Directory [Member]                      
Revenue from External Customer [Line Items]                      
Total revenue                 $ 44,651 $ 58,067 $ 88,531