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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 |  |

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