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Shareholders' Equity (Narrative) (Details) - $ / shares shares in Millions | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Common Class A [Member] |  |
Class of Stock [Line Items] |  |
Common Stock, par value per share | $ 0.001 | $ 0.001
Common Stock reserved for the conversion of Class B Common Stock and the exercise of stock options | 3.3 |
Number of shares authorized to be repurchased | 2.0 |
Common Class B [Member] |  |
Class of Stock [Line Items] |  |
Common Stock, par value per share | $ 0.001 | $ 0.001

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