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Summary of Significant Accounting Policies (Share-Based Payment) (Details) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2017 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Net Cash Provided by (Used in) Operating Activities |  | $ 101,112 | $ 114,571 | $ 98,154
Accounting Standards Update 2016-09, Statutory Tax Withholding Component [Member] |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Effective Income Tax Rate Reconciliation, Share-based Compensation, Excess Tax Benefit, Amount | $ 500 |  |  |
Net Cash Provided by (Used in) Operating Activities |  | $ 818 | $ 499 | $ 897

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