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Regulatory matters - Regulatory Assets and Liabilities (Detail) - USD ($)
$ in Thousands
Jun. 30, 2019
Dec. 31, 2018
Regulatory Assets And Liabilities [Line Items]    
Total regulatory assets $ 446,723 $ 450,474
Less: current regulatory assets (42,398) (59,037)
Non-current regulatory assets 404,325 391,437
Total regulatory liabilities 581,897 578,592
Less: current regulatory liabilities (38,880) (39,005)
Non-current regulatory liabilities 543,017 539,587
Taxes    
Regulatory Assets And Liabilities [Line Items]    
Total regulatory liabilities 322,290 323,384
Cost of removal    
Regulatory Assets And Liabilities [Line Items]    
Total regulatory liabilities 196,469 193,564
Rate-base offset    
Regulatory Assets And Liabilities [Line Items]    
Total regulatory liabilities 9,778 10,900
Fuel and commodity costs adjustment    
Regulatory Assets And Liabilities [Line Items]    
Total regulatory liabilities 19,715 22,552
Deferred compensation received in relation to lost production    
Regulatory Assets And Liabilities [Line Items]    
Total regulatory liabilities 4,788 6,003
Deferred construction costs - fuel related    
Regulatory Assets And Liabilities [Line Items]    
Total regulatory liabilities 7,178 7,258
Pension and post-employment benefits    
Regulatory Assets And Liabilities [Line Items]    
Total regulatory liabilities 6,519 2,170
Other    
Regulatory Assets And Liabilities [Line Items]    
Total regulatory liabilities 15,160 12,761
Environmental remediation    
Regulatory Assets And Liabilities [Line Items]    
Total regulatory assets 83,903 82,295
Pension and post-employment benefits    
Regulatory Assets And Liabilities [Line Items]    
Total regulatory assets 125,412 125,959
Debt premium    
Regulatory Assets And Liabilities [Line Items]    
Total regulatory assets 45,499 48,847
Fuel and commodity costs adjustment    
Regulatory Assets And Liabilities [Line Items]    
Total regulatory assets 21,283 26,310
Rate adjustment    
Regulatory Assets And Liabilities [Line Items]    
Total regulatory assets 33,516 37,202
Clean Energy and other customer programs    
Regulatory Assets And Liabilities [Line Items]    
Total regulatory assets 24,330 22,269
Deferred construction costs - fuel related    
Regulatory Assets And Liabilities [Line Items]    
Total regulatory assets 13,807 13,986
Asset retirement    
Regulatory Assets And Liabilities [Line Items]    
Total regulatory assets 22,529 21,048
Taxes    
Regulatory Assets And Liabilities [Line Items]    
Total regulatory assets 34,971 34,822
Rate case costs    
Regulatory Assets And Liabilities [Line Items]    
Total regulatory assets 8,040 7,990
Other    
Regulatory Assets And Liabilities [Line Items]    
Total regulatory assets $ 33,433 $ 29,746