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Income Taxes - Additional Information (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Schedule Of Income Taxes [Line Items]      
Income tax expense due to re-measurement of deferred tax assets and liabilities as result of changes in federal tax rate $ 13,400    
Increase in income tax expense for re-measurement due to change in corporate tax rate 13,400    
Valuation allowance related to gross deferred tax assets 3,598 $ 3,114  
Deferred tax assets, net operating loss carry forwards $ 36,000    
Unrecognized tax benefits, accrued interest   $ 100 $ 100
Minimum      
Schedule Of Income Taxes [Line Items]      
Deferred tax assets, net operating loss carry forwards, expiration date 2020    
Maximum      
Schedule Of Income Taxes [Line Items]      
Deferred tax assets, net operating loss carry forwards, expiration date 2037    
PENNSYLVANIA      
Schedule Of Income Taxes [Line Items]      
Valuation allowance related to gross deferred tax assets $ 3,600