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Other long-term liabilities - Schedule of Long-Term Liabilities and Deferred Credits (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Oct. 01, 2019
Jan. 01, 2019
Dec. 31, 2017
Other Long-term Liabilities          
Advances in aid of construction (a) $ 60,828 $ 63,703      
Environmental remediation obligation (b) 58,061 55,621     $ 54,322
Asset retirement obligations (c) 53,879 43,291     $ 44,166
Customer deposits (d) 31,946 29,974      
Unamortized investment tax credits (e) 18,234 17,491      
Deferred credits (f) 18,952 42,711      
Preferred shares, Series C (g) 13,793 13,418      
Lease liabilities (note 1(q)) 9,695   $ 1,316 $ 8,295  
Lease liabilities (note 1(q))   3,436      
Other (h) 35,952 28,360      
Other Liabilities 301,340 298,005      
Less: current portion (57,939) (42,337)      
Other long-term liabilities 243,401 255,668      
Transfers from advances in aid of construction to contributions in aid of construction 5,465 3,687      
Estimated environmental remediation costs 58,484 59,181      
Accrual for environmental loss contingencies to be incurred over next four years $ 36,382        
Accrual for environmental loss contingencies, payment period 31 years        
Amount of regulatory assets $ 559,887 460,095      
Minimum          
Other Long-term Liabilities          
Other liability repayment period 5 years        
Accrual environmental cost 1.70%        
Maximum          
Other Long-term Liabilities          
Other liability repayment period 40 years        
Accrual environmental cost 2.10%        
Environmental costs          
Other Long-term Liabilities          
Regulatory asset, expenditure recovery provided by regulator, time period 7 years        
Amount of regulatory assets $ 82,300 $ 82,295      
Series C Preferred Stock          
Other Long-term Liabilities          
Preferred shares, Series C (g) $ 13,793