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Regulatory matters - Regulatory Assets and Liabilities (Detail) - USD ($)
$ in Thousands
 6 Months Ended 12 Months Ended
Jun. 30, 2018
Dec. 31, 2018
Dec. 31, 2017
Regulatory Liabilities [Line Items]      
Total regulatory assets   $ 450,474 $ 441,526
Less: current regulatory assets   (59,037) (66,567)
Non-current regulatory assets   391,437 374,959
Total regulatory liabilities   578,592 576,124
Less: current regulatory liabilities   (39,005) (37,687)
Non-current regulatory liabilities   539,587 538,437
Reduction of regulatory asset $ 15,586   327,947
Increase of regulatory liability     327,947
Taxes      
Regulatory Liabilities [Line Items]      
Total regulatory liabilities   323,384 321,138
Cost of removal      
Regulatory Liabilities [Line Items]      
Total regulatory liabilities   193,564 184,188
Rate-base offset      
Regulatory Liabilities [Line Items]      
Total regulatory liabilities   10,900 13,214
Fuel and commodity costs adjustment      
Regulatory Liabilities [Line Items]      
Total regulatory liabilities   23,517 23,543
Deferred compensation received in relation to lost production      
Regulatory Liabilities [Line Items]      
Total regulatory liabilities   6,897 9,398
Deferred construction costs - fuel related      
Regulatory Liabilities [Line Items]      
Total regulatory liabilities   7,258 7,418
Pension and post-employment benefits      
Regulatory Liabilities [Line Items]      
Total regulatory liabilities   877 10,082
Other      
Regulatory Liabilities [Line Items]      
Total regulatory liabilities   $ 12,195 7,143
Rate adjustment      
Regulatory Liabilities [Line Items]      
Collection period for services rendered   24 months  
Minimum | Rate-base offset      
Regulatory Liabilities [Line Items]      
Amortization period for regulatory liability   10 years  
Maximum | Rate-base offset      
Regulatory Liabilities [Line Items]      
Amortization period for regulatory liability   16 years  
Environmental remediation      
Regulatory Liabilities [Line Items]      
Total regulatory assets   $ 82,295 82,711
Regulatory assets recovery period   7 years  
Pension and post-employment benefits      
Regulatory Liabilities [Line Items]      
Total regulatory assets   $ 125,959 105,712
Regulatory asset approved not yet being recovered, average recovery term   10 years  
Debt premium      
Regulatory Liabilities [Line Items]      
Total regulatory assets   $ 48,847 57,406
Fuel and commodity costs adjustment      
Regulatory Liabilities [Line Items]      
Total regulatory assets   26,310 34,525
Rate adjustment      
Regulatory Liabilities [Line Items]      
Total regulatory assets   36,484 35,813
Clean Energy and other customer programs      
Regulatory Liabilities [Line Items]      
Total regulatory assets   22,269 20,582
Deferred construction costs - fuel related      
Regulatory Liabilities [Line Items]      
Total regulatory assets   13,986 14,344
Asset retirement      
Regulatory Liabilities [Line Items]      
Total regulatory assets   21,048 16,080
Taxes      
Regulatory Liabilities [Line Items]      
Total regulatory assets   34,822 36,546
Rate case costs      
Regulatory Liabilities [Line Items]      
Total regulatory assets   7,990 9,295
Other      
Regulatory Liabilities [Line Items]      
Total regulatory assets   $ 30,464 $ 28,512