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Capital Stock (Details) - USD ($) $ in Millions | 6 Months Ended |
--- | --- | ---
Jun. 30, 2018 | Dec. 31, 2017
Class of Stock [Line Items] |  |
Share-based Compensation Arrangement by Share-based Payment Award, Aggregate Value of Shares Issued | $ 0.5 |
Class A Common Stock |  |
Class of Stock [Line Items] |  |
Common stock outstanding (in shares) | 34,200,792 | 33,560,202
Stock repurchased and retired (in shares) | 0 |
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period | 11,389 |
Class B Common Stock |  |
Class of Stock [Line Items] |  |
Common stock outstanding (in shares) | 22,559,402 | 22,905,357

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