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Employee Benefit Plans (Details) (Class A common stock, USD $)
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12 Months Ended | ||
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Dec. 31, 2013
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Dec. 31, 2012
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Dec. 31, 2011
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| Employee benefit plans | |||
| Number of shares that remain available for issuance | 4,600,000 | ||
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Employee Stock Purchase Plan
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| Employee benefit plans | |||
| Number of shares authorized for issue | 2,500,000 | ||
| Number of shares that remain available for issuance | 1,400,000 | ||
| Minimum period for which full-time employees employed by the entity are eligible to participate in the ESPP | 3 months | ||
| Maximum fair value of capital stock permitted to be purchased by employees in any one year under ESPP | $ 25,000 | ||
| Purchase price as percentage of closing market price on the last business day of each calendar quarter under ESPP | 85.00% | ||
| Number of shares of common stock purchased under ESPP | 268,000 | 158,000 | 140,000 |