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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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Number of options outstanding, including both vested and non-vested options.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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Period from grant date that an equity-based award expires, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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Stock-Based Compensation - Stock Incentive Plans (Details) - shares	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Class A common stock
Stock-Based Compensation
Number of common stock available for future grant under stock incentive plans (in shares)	4,300,000
Restricted stock units
Stock-Based Compensation
Restricted stock units (in shares)	57,328	96,768	121,877	151,683
Stock options
Stock-Based Compensation
Stock options (in shares)	5,893,241	6,669,614	6,271,058	7,908,300
Stock options \| Minimum
Stock-Based Compensation
Vesting period	3 years
Stock options \| Maximum
Stock-Based Compensation
Expiration term	10 years
Vesting period	5 years