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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, 2018
Dec. 31, 2017
Dec. 31, 2016
Other Long-term Liabilities      
Advances in aid of construction (a) $ 63,703 $ 62,683  
Environmental remediation obligation (b) 55,621 54,322 $ 47,202
Asset retirement obligations (c) 43,291 44,166 $ 18,486
Customer deposits (d) 29,974 28,529  
Unamortized investment tax credits (e) 17,491 17,839  
Deferred credits (f) 42,711 21,168  
Preferred shares, Series C (g) 13,418 14,718  
Other (h) 39,710 45,434  
Other Liabilities 305,919 288,859  
Less: current portion (42,337) (46,754)  
Other long-term liabilities 263,582 242,105  
Transfers from advances in aid of construction to contributions in aid of construction 3,687 10,498  
Estimated environmental remediation costs 59,181 57,292  
Accrual for environmental loss contingencies to be incurred over next three years $ 36,611    
Accrual for environmental loss contingencies, payment period 27 years    
Amount of regulatory assets $ 450,474 441,526  
Minimum      
Other Long-term Liabilities      
Other liability repayment period 5 years    
Accrual environmental cost 2.50%    
Maximum      
Other Long-term Liabilities      
Other liability repayment period 40 years    
Accrual environmental cost 2.80%    
Environmental costs      
Other Long-term Liabilities      
Regulatory asset, expenditure recovery provided by regulator, time period 7 years    
Amount of regulatory assets $ 82,295 $ 82,711  
Series C Preferred Stock      
Other Long-term Liabilities      
Preferred shares, Series C (g) $ 13,418