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Income Taxes - Tax credit carryforward (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Tax credit carryforward    
Income tax receivable $ 3,914 $ 8,371
Federal    
Tax credit carryforward    
Decrease in alternative minimum tax credit credit carryforward 4,500  
Alternative minimum tax credit carryforward $ 3,900  
ATM credit carryforward | Federal    
Tax credit carryforward    
Income tax receivable   $ 8,400