Business combinations and disposal - Pro forma information (Details) - USD ($) $ / shares in Units, $ in Thousands  | 
12 Months Ended | |
|---|---|---|
Sep. 30, 2019  | 
Sep. 30, 2018  | 
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| Business Combinations [Abstract] | ||
| Revenue | $ 2,985,244 | $ 3,016,823 | 
| Net income | $ 243,968 | $ 218,647 | 
| Basic earnings per share attributable to MAXIMUS (in dollars per share) | $ 3.79 | $ 3.34 | 
| Diluted earnings per share attributable to MAXIMUS (in shares) | $ 3.77 | $ 3.32 | 
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- Definition The pro forma basic net income per share for a period as if the business combination or combinations had been completed at the beginning of a period. Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef 
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- Definition The pro forma diluted net income per share for a period as if the business combination or combinations had been completed at the beginning of a period. Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef 
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- Definition The pro forma net Income or Loss for the period as if the business combination or combinations had been completed at the beginning of a period. Reference 1: http://www.xbrl.org/2003/role/disclosureRef 
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- Definition The pro forma revenue for a period as if the business combination or combinations had been completed at the beginning of the period. Reference 1: http://www.xbrl.org/2003/role/disclosureRef 
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- References No definition available. 
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