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Derivative Instruments - Effect of Cash Flow Hedges on Income Statement (Details) - USD ($)
$ in Thousands
3 Months Ended
Apr. 30, 2019
Apr. 30, 2018
Derivative Instruments, Gain (Loss) [Line Items]    
Total revenues $ 825,055 $ 618,643
Revenues    
Derivative Instruments, Gain (Loss) [Line Items]    
Amount of gains (losses) related to foreign currency forward contracts designated as cash flow hedges $ 16