| 
           Credit facilities (Details) 
        In Millions, unless otherwise specified  | 
        0 Months Ended | 3 Months Ended | 6 Months Ended | 6 Months Ended | |||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 
           Mar. 15, 2013
 
          Credit Facility
 
          USD ($) 
         | 
        
           Mar. 31, 2014
 
          Credit Facility
 
          USD ($) 
         | 
        
           Mar. 31, 2014
 
          Credit Facility
 
          Base Rate 
         | 
        
           Mar. 31, 2014
 
          Credit Facility
 
          Euro currency Rate 
         | 
        
           Mar. 31, 2014
 
          Credit Facility
 
          Index Rate 
         | 
        
           Mar. 31, 2014
 
          Credit Facility
 
          Minimum 
         | 
        
           Mar. 31, 2014
 
          Credit Facility
 
          Minimum
 
          Base Rate 
         | 
        
           Mar. 31, 2014
 
          Credit Facility
 
          Minimum
 
          Euro currency Rate 
         | 
        
           Mar. 31, 2014
 
          Credit Facility
 
          Minimum
 
          Index Rate 
         | 
        
           Mar. 31, 2014
 
          Credit Facility
 
          Maximum 
         | 
        
           Mar. 31, 2014
 
          Credit Facility
 
          Maximum
 
          Base Rate 
         | 
        
           Mar. 31, 2014
 
          Credit Facility
 
          Maximum
 
          Euro currency Rate 
         | 
        
           Mar. 31, 2014
 
          Credit Facility
 
          Maximum
 
          Index Rate 
         | 
        
           Mar. 15, 2013
 
          Swingline loans
 
          USD ($) 
         | 
        
           Mar. 31, 2014
 
          Letters of credit
 
          USD ($)
 
          Letter 
         | 
        
           Mar. 15, 2013
 
          Letters of credit
 
          USD ($) 
         | 
        
           Mar. 31, 2014
 
          Letters of credit
 
          Minimum 
         | 
        
           Mar. 31, 2014
 
          Letters of credit
 
          Maximum 
         | 
        
           Mar. 31, 2014
 
          Letter of credit (Other than Revolving Credit Agreement)
 
          USD ($)
 
          Letter 
         | 
        
           Mar. 31, 2014
 
          Atlantic Innovation Fund of Canada
 
          USD ($)
 
          Installment 
         | 
        
           Mar. 31, 2014
 
          Atlantic Innovation Fund of Canada
 
          CAD 
         | 
        
           Mar. 31, 2014
 
          Performance Bonds
 
          USD ($) 
         | 
        
           Sep. 30, 2013
 
          Performance Bonds
 
          USD ($) 
         | 
      |
| Credit facilities | |||||||||||||||||||||||
| Term of the credit agreement | 5 years | ||||||||||||||||||||||
| Maximum borrowing capacity | $ 100.0 | $ 5.0 | $ 30.0 | 1.8 | |||||||||||||||||||
| Number of letters of credit issued | 4 | 2 | |||||||||||||||||||||
| Amount borrowed | 15 | ||||||||||||||||||||||
| Amount borrowed | 6.7 | 3.0 | |||||||||||||||||||||
| Amount repaid | 15 | ||||||||||||||||||||||
| Leverage ratio | 2.5 | ||||||||||||||||||||||
| Leverage ratio, actual | 0.1 | ||||||||||||||||||||||
| Leverage ratio to calculate annual commitment fee (as a percent) | 0.15% | 0.30% | |||||||||||||||||||||
| Variable rate basis | Base Rate | Eurocurrency Rate | Index Rate | ||||||||||||||||||||
| Margin (as a percent) | 0.00% | 1.00% | 1.00% | 0.75% | 1.75% | 1.75% | |||||||||||||||||
| Interest charges (as a percent) | 0.00% | 0.00% | |||||||||||||||||||||
| Amount outstanding | 1.3 | 1.4 | |||||||||||||||||||||
| Number of remaining quarterly installments | 33 | 33 | |||||||||||||||||||||
| Performance bond commitments | $ 50.5 | $ 50.8 | |||||||||||||||||||||