Annual report pursuant to Section 13 and 15(d)

Business combinations and disposals - Schedule of Assets and Liabilities Recorded in the Company's Financial Statements (Details)

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Business combinations and disposals - Schedule of Assets and Liabilities Recorded in the Company's Financial Statements (Details) - USD ($)
$ in Thousands
12 Months Ended
Feb. 29, 2016
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2016
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net [Abstract]        
Goodwill   $ 399,882 $ 402,976 $ 397,558
Ascend Management Innovations LLC        
Business Combination, Consideration Transferred        
Purchase consideration, net of cash acquired $ 44,100 44,069    
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net [Abstract]        
Billed and unbilled receivables   4,069    
Other assets   407    
Property and equipment and other assets   707    
Deferred income taxes   557    
Intangible assets 22,300 22,300    
Total identifiable assets acquired   28,040    
Accounts payable and other liabilities   1,414    
Deferred revenue   554    
Total liabilities assumed   1,968    
Net identifiable assets acquired   26,072    
Goodwill $ 18,000 17,997    
Net assets acquired   $ 44,069