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Regulatory matters - Approved Annual Revenue Increases (Details)
CAD in Thousands, $ in Thousands
1 Months Ended
Apr. 12, 2017
USD ($)
Dec. 08, 2016
USD ($)
Nov. 21, 2016
USD ($)
Jun. 30, 2016
USD ($)
Feb. 18, 2016
USD ($)
Feb. 10, 2016
USD ($)
May 31, 2017
CAD
Apr. 30, 2017
USD ($)
New Hampshire Public Utilities Commission                
Regulatory Liabilities [Line Items]                
Approved revenue increase $ 6,750     $ 6,105        
Massachusetts Department of Public Utilities                
Regulatory Liabilities [Line Items]                
Approved rate increase               $ 2,928
Illinois Commerce Commission                
Regulatory Liabilities [Line Items]                
Approved rate increase | CAD             CAD 2,200  
Arizona Corporate Commission                
Regulatory Liabilities [Line Items]                
Approved revenue increase     $ 1,935   $ 2,725      
California Public Utilities Commission                
Regulatory Liabilities [Line Items]                
Approved revenue increase   $ 8,318   2,175        
Various                
Regulatory Liabilities [Line Items]                
Approved revenue increase       $ 3,551        
Annual Revenue Increase Approval, March 2016 | Massachusetts Department of Public Utilities                
Regulatory Liabilities [Line Items]                
Approved revenue increase           $ 8,300