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Stockholders' Equity (Narrative) (Details) - Class A common stock - USD ($) | 9 Months Ended | 20 Months Ended |
--- | --- | --- | ---
Sep. 30, 2017 | Sep. 30, 2017 | Feb. 29, 2016
Equity, Class of Treasury Stock [Line Items] |  |  |
Authorized repurchase amount |  |  | $ 250,000,000
Restricted stock units (RSUs) |  |  |
Equity, Class of Treasury Stock [Line Items] |  |  |
Repurchase related to stock award tax withholdings (in shares) | 32,300 |  |
Average purchase price per share (in dollars per share) | $ 99.33 |  |
Repurchase related to stock award tax withholdings | $ 3,200,000 |  |
2016 Share Repurchase Authorization |  |  |
Equity, Class of Treasury Stock [Line Items] |  |  |
Remaining authorized repurchase amount | $ 164,800,000 | $ 164,800,000 |
Average purchase price per share (in dollars per share) | $ 91.33 | $ 83.25 |

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- Definition The amount of adjustment to stockholders' equity associated with an employee's income tax withholding obligation as part of a net-share settlement of a share-based award. + References No definition available. + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Definition For net-share settlement of share-based awards when the employer settles employees' income tax withholding obligations, this element represents the number of shares the employees use to repay the employer. + References No definition available. + Details
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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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