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Income Taxes - Effective Income Tax Rate Varied from Statutory Guernsey Tax Rate (Detail)
12 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2015
Income Tax Disclosure [Abstract]      
Statutory Guernsey tax rate 0.00% 0.00% 0.00%
Foreign taxes (percentage) 15.00% 16.00% 13.00%
Effective income tax rate 15.00% 16.00% 13.00%