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Stockholders' Equity (Narrative) (Details) - USD ($)	6 Months Ended
	Jun. 30, 2018	Feb. 29, 2016
Equity, Class of Treasury Stock [Line Items]
Repurchase related to stock award tax withholdings (in shares)	29,710
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)	$ 112.36
Repurchase related to stock award tax withholdings	$ 3,300,000
2016 Share Repurchase Authorization | Class A common stock
Equity, Class of Treasury Stock [Line Items]
Remaining authorized repurchase amount	$ 98,600,000

X

- 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

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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

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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

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- Definition

Amount of stock repurchase plan authorized.

+ References

No definition available.

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- Definition

Amount remaining of a stock repurchase plan authorized.

+ References

No definition available.

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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

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