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Income Taxes (Details) - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended | 12 Months Ended
--- | --- | --- | ---
Jun. 30, 2018 | Jun. 30, 2017 | Mar. 31, 2017 | Jun. 30, 2018 | Jun. 30, 2017 | Dec. 31, 2017
Operating Loss Carryforwards [Line Items] |  |  |  |  |  |
Federal tax rate, percent |  |  |  | 21.00% |  | 35.00%
Income tax benefit from the Tax Act |  |  |  |  |  | $ 39,300
Income tax provision | $ 26,529 | $ 33,019 |  | $ 47,162 | $ 59,865 |
Effective income tax rates | 27.80% | 37.50% |  | 27.70% | 37.70% |
Actual income tax rates | 27.20% | 36.80% |  | 27.10% | 35.30% |
Unrecognized income tax benefits | $ 800 |  |  | $ 800 |  | 800
Accrued interest expense related to unrecognized income tax benefits | 100 |  |  | 100 |  | 100
Interest income related to unrecognized income tax benefits |  |  |  |  | $ 300 |
Interest expense related to unrecognized income tax benefits | 100 | $ 100 |  | 100 |  |
Unrecognized tax benefits, decrease resulting from settlements with taxing authorities |  |  | $ 3,300 |  |  |
Other Long-term Liabilities [Member] |  |  |  |  |  |
Operating Loss Carryforwards [Line Items] |  |  |  |  |  |
Income tax reserves | $ 900 |  |  | $ 900 |  | $ 900

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