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Long-Term Debt - 5.25% Senior Notes (Details) - 5.25% Senior Note - USD ($) | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 08, 2015
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Debt Instrument [Line Items] |  |  |
Interest rate | 5.25% |  | 5.25%
Face amount | $ 300,000,000 | $ 300,000,000 | $ 300,000,000
Unamortized underwriters' fees, discount and debt issuance costs | $ 3,300,000 |  |

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