| 
           Employee benefit plans and deferred compensation (Details) (USD $) 
        In Millions, unless otherwise specified  | 
        12 Months Ended | ||
|---|---|---|---|
| 
           Sep. 30, 2014 
         | 
        
           Sep. 30, 2013 
         | 
        
           Sep. 30, 2012 
         | 
      |
| Employee benefit plans and deferred compensation | |||
| Employer contribution to 401(k) plans | $ 4.3 | $ 3.8 | $ 3.0 | 
| Investments in mutual funds within the rabbi trust | $ 6.2 | ||
| X | ||||||||||
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- Details 
          
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| X | ||||||||||
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- Definition 
          The amount of the cost recognized during the period for defined contribution plans. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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| X | ||||||||||
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- Definition 
          Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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