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Long-term Debt (Details) $ in Thousands | 3 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | ---
Dec. 31, 2017 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2014 USD ($) | Feb. 06, 2018 USD ($) | Dec. 31, 2016 USD ($) | Jan. 07, 2016 USD ($)
Line of Credit Facility [Line Items] |  |  |  |  |  |
Line of credit facility | $ 200,000 | $ 200,000 |  |  |  |
Percentage of subsidiary stock securing debt (in hundredths) |  | 0.65 |  |  |  |
Unused commitment fee on the average daily unused portion (in hundredths) |  | 0.25% |  |  |  |
Average interest rate for the year |  | 2.99% |  |  |  |
Letters of Credit Outstanding, Amount | 1,000 | $ 1,000 |  |  |  |
Long-term debt | 104,400 | $ 104,400 |  |  | $ 104,400 | $ 104,400
Special dividend | $ 41,700 |  | $ 50,700 |  |  |
Alternate base rates, applicable margins [Abstract] |  |  |  |  |  |
Applicable margin, federal funds rate (in hundredths) | 0.50% | 0.50% |  |  |  |
Applicable margin, 30-day LIBOR (in hundredths) | 2.00% | 2.00% |  |  |  |
LIBOR Borrowings [Member] |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |
Description of basis for variable rate |  | LIBOR |  |  |  |
Applicable margin on variable rate on borrowings, minimum (in hundredths) | 1.50% | 1.50% |  |  |  |
Applicable margin on variable rate on borrowings, maximum (in hundredths) | 2.25% | 2.25% |  |  |  |
Alternate Base Rate Borrowings [Member] |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |
Description of basis for variable rate |  | alternate base rate |  |  |  |
Applicable margin on variable rate on borrowings, minimum (in hundredths) | 0.00% | 0.00% |  |  |  |
Applicable margin on variable rate on borrowings, maximum (in hundredths) | 0.50% | 0.50% |  |  |  |
Subsequent Event [Member] |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |
Line of credit facility |  |  |  | $ 350,000 |  |
Maximum borrowing capacity |  |  |  | $ 400,000 |  |

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