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Stockholders' Equity (Narrative) (Details) - USD ($) | 9 Months Ended |
--- | --- | ---
Sep. 30, 2018 | Feb. 29, 2016
Equity, Class of Treasury Stock [Line Items] |  |
Repurchase related to stock award tax withholdings (in shares) | 29,875 |
Class A common stock |  |
Equity, Class of Treasury Stock [Line Items] |  |
Authorized repurchase amount |  | $ 250,000,000
Class A common stock | Restricted stock units (RSUs) |  |
Equity, Class of Treasury Stock [Line Items] |  |
Average purchase price per share (in dollars per share) | $ 112.19 |
Repurchase related to stock award tax withholdings | $ 3,400,000 |
2016 Share Repurchase Authorization | Class A common stock |  |
Equity, Class of Treasury Stock [Line Items] |  |
Remaining authorized repurchase amount | $ 50,900,000 |

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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://fasb.org/us-gaap/role/ref/legacyRef -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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