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Summary of Significant Accounting Policies New Accounting Pronouncements (Details) - USD ($) $ in Thousands | Mar. 31, 2018 | Jan. 19, 2018 | Dec. 31, 2017 | Dec. 22, 2017
--- | --- | --- | --- | ---
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Senior Notes |  | $ 100,000 |  | $ 300,000
Accounting Standards Update 2014-09 [Member] |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Cumulative Effect of New Accounting Principle in Period of Adoption |  |  | $ 10,900 |
Accounting Standards Update 2016-16 [Member] |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Cumulative Effect of New Accounting Principle in Period of Adoption |  |  | 900 |
Senior Notes [Member] |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Senior Notes | $ 400,000 |  | $ 300,000 |
Debt Instrument, Interest Rate, Stated Percentage |  |  | 5.00% |

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