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Stockholders' Equity (Narrative) (Details) | 1 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | ---
Dec. 31, 2017 USD ($) | Dec. 31, 2017 USD ($) $ / shares shares | Dec. 31, 2016 shares | Dec. 31, 2015 shares | Feb. 29, 2016 USD ($) | Jul. 20, 2012 shares | Aug. 31, 2011 shares
Class of Stock [Line Items] |  |  |  |  |  |  |
Shares repurchased in association with tax withholdings on the vesting of RSUs (in shares) | shares |  | 32,457 | 94,826 | 77,649 |  |  |
Net premiums paid | $ (33,400,000) |  |  |  |  |  |
Class A common stock |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |
Conversion of common stock (in shares) | shares |  | 1 |  |  |  |  |
Common stock number of votes |  | 1 |  |  |  |  |
Number of shares authorized to be repurchased (in shares) | shares |  |  |  |  |  | 1,000,000 | 2,000,000
Amount authorized to be repurchased |  |  |  |  | $ 250,000,000 |  |
Class A common stock | Restricted Stock Units (RSUs) |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |
Treasury stock acquired, average cost (in dollars per share) | $ / shares |  | $ 99.40 |  |  |  |  |
Adjustments related to tax withholding for share-based compensation |  | $ 3,200,000 |  |  |  |  |
Class A common stock | The 2016 Stock Repurchase Authorization |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |
Remaining authorized repurchase amount | $ 162,600,000 | $ 162,600,000 |  |  |  |  |
Class B common stock |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |
Percentage of Class B common to total common stock subject to automatic conversion |  | 1.00% |  |  |  |  |
Common stock number of votes |  | 10 |  |  |  |  |

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- Definition Total cost of shares repurchased divided by the total number of shares repurchased. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 30 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6405813&loc=d3e23239-112655 + Details
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