| 
           Credit facilities (Details) 
        In Millions, unless otherwise specified  | 
        0 Months Ended | 6 Months Ended | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| 
           Mar. 15, 2013
 
          Credit Facility
 
          USD ($) 
         | 
        
           Mar. 31, 2013
 
          Credit Facility
 
          Minimum 
         | 
        
           Mar. 31, 2013
 
          Credit Facility
 
          Maximum 
         | 
        
           Mar. 15, 2013
 
          Swingline loans
 
          USD ($) 
         | 
        
           Mar. 31, 2013
 
          Letters of credit
 
          USD ($)
 
          Item 
         | 
        
           Mar. 15, 2013
 
          Letters of credit
 
          USD ($) 
         | 
        
           Mar. 31, 2013
 
          Letter of credit (Other than Revolving Credit Agreement)
 
          USD ($)
 
          Item 
         | 
        
           Mar. 31, 2013
 
          Atlantic Innovation Fund of Canada
 
          USD ($)
 
          Item 
         | 
        
           Mar. 31, 2013
 
          Atlantic Innovation Fund of Canada
 
          CAD 
         | 
        
           Mar. 31, 2013
 
          Performance Bonds
 
          USD ($) 
         | 
        
           Sep. 30, 2012
 
          Performance Bonds
 
          USD ($) 
         | 
      |
| Credit facilities | |||||||||||
| Term of the credit agreement | 5 years | ||||||||||
| Maximum borrowing capacity | $ 100 | $ 5 | $ 30 | ||||||||
| Number of letters of credit issued | 5 | 2 | |||||||||
| Amount outstanding | 15.5 | 3.0 | |||||||||
| Leverage ratio | 2.5 | ||||||||||
| Leverage ratio, actual | 0.5 | ||||||||||
| Maximum borrowing capacity under loan | 1.8 | ||||||||||
| Interest charges | 0.00% | 0.00% | |||||||||
| Number of remaining quarterly installments | 37 | 37 | |||||||||
| Amount outstanding | 1.6 | 1.6 | |||||||||
| Performance bond commitments | $ 47.4 | $ 48.0 | |||||||||