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Summary of Significant Accounting Policies - Share Repurchase Program (Details) - USD ($) | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Feb. 28, 2017 | Apr. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Treasury stock, shares, acquired | 218,000 | 575,000 |  |  |
Treasury stock, value, acquired | $ 1,839,000 | $ 3,412,000 | $ 6,387,000 |  |
Repurchase fees | $ 9,000 | $ 23,000 |  |  |
Treasury Stock |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Treasury stock, shares, acquired | 217,774 | 574,848 | 1,298,418 |  |
Treasury stock, value, acquired | $ 1,839,000 | $ 3,412,000 | $ 6,387,000 |  |
Class A Common Stock |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Stock repurchase program, authorized amount (up to) |  |  |  | $ 15,000,000 | $ 10,000,000

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