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Income Taxes Additional Information (Details) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Dec. 31, 2019
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Income Tax Contingency [Line Items]        
Document Period End Date   Dec. 31, 2019    
Deferred tax liabilities, period increase (decrease)   $ 115,947    
Unrecognized tax benefits before impact of deductions for state taxes, excluding interest and penalties $ 4,037 4,037 $ 1,411  
Unrecognized tax benefits that would impact effective tax rate 27 27 70  
Accrued interest expense net of the benefit of tax deductions 27 27    
Cumulative reduction in unrecognized tax benefits, due to lapsing of statutes of limitations   43    
Other 4,148 4,148 1,138  
Provision for income taxes   8,454 10,065 $ 35,393
Tax Cuts and Jobs Act, Incomplete Accounting, Change in Tax Rate, Deferred Tax Asset, Provisional Income Tax Expense     2,357  
Tax Cuts and Jobs Act, Measurement Period Adjustment, Income Tax Expense (Benefit) 67      
Tax Cuts And Jobs Act Of 2017, Measurement Period Adjustment, Increase (Decrease) Of Regulatory Liability 455      
Liability recorded in tax memorandum account     6,504  
Deferred Income Tax Charge        
Income Tax Contingency [Line Items]        
Deferred tax liabilities, period increase (decrease)   109,435    
Tax Cuts and Jobs Act, Incomplete Accounting, Change in Tax Rate, Deferred Tax Asset, Provisional Income Tax Expense     $ 83,666  
Federal        
Income Tax Contingency [Line Items]        
Operating Loss Carryforwards 9,213 9,213    
Connecticut        
Income Tax Contingency [Line Items]        
Operating Loss Carryforwards 32,604 32,604    
Tax Credit Carryforward, Amount 1,000 1,000    
Maine        
Income Tax Contingency [Line Items]        
Operating Loss Carryforwards $ 9,506 $ 9,506