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Income Taxes Additional Information (Details) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Dec. 31, 2018
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Income Tax Contingency [Line Items]        
Deferred tax liabilities, period increase (decrease)   $ (6,144)    
Unrecognized tax benefits before impact of deductions for state taxes, excluding interest and penalties $ 1,411 1,411 $ 1,359  
Unrecognized tax benefits that would impact effective tax rate 70 70 46  
Accrued interest expense net of the benefit of tax deductions 70 70    
Cumulative reduction in unrecognized tax benefits, due to lapsing of statutes of limitations   24    
Other 968 968 1,041  
Provision for income taxes   10,065 35,393 $ 33,542
Tax Cuts And Jobs Act Of 2017, Incomplete Accounting, Change In Tax Rate, Deferred Tax Asset, Provisional Income Tax Expense     2,357  
Tax Cuts And Jobs Act of 2017, Measurement Period Adjustment, Income Tax Expense (Benefit) 67      
Tax Cuts And Jobs Act Of 2017, Measurement Period Adjustment, Increase (Decrease) Of Regulatory Liability 455      
Deferred Income Tax Charge        
Income Tax Contingency [Line Items]        
Deferred tax liabilities, period increase (decrease)   3,342    
Tax Cuts And Jobs Act Of 2017, Incomplete Accounting, Change In Tax Rate, Deferred Tax Asset, Provisional Income Tax Expense     83,666  
2017 Federal and State Repairs and Maintenance Deduction [Member]        
Income Tax Contingency [Line Items]        
Income tax credits and adjustments   12,873    
2017 Federal and State Repairs and Maintenance Deduction [Member] | Domestic Tax Authority        
Income Tax Contingency [Line Items]        
Other $ 2,703 2,703    
2017 Federal and State Repairs and Maintenance Deduction [Member] | State and Local Jurisdiction        
Income Tax Contingency [Line Items]        
Provision for income taxes   $ (899)    
2016 Federal and State Repairs and Maintenance Deduction [Member]        
Income Tax Contingency [Line Items]        
Income tax credits and adjustments     20,168  
2016 Federal and State Repairs and Maintenance Deduction [Member] | Domestic Tax Authority        
Income Tax Contingency [Line Items]        
Other     7,059  
2016 Federal and State Repairs and Maintenance Deduction [Member] | State and Local Jurisdiction        
Income Tax Contingency [Line Items]        
Provision for income taxes     $ (1,159)