Employee Benefit Plans - Employee Stock Purchase Plan (Details) - Class A common stock - USD ($) |
12 Months Ended | ||
|---|---|---|---|
Dec. 31, 2015 |
Dec. 31, 2014 |
Dec. 31, 2013 |
|
| Employee benefit plans | |||
| Number of shares that remain available for issuance | 4,300,000 | ||
| Employee Stock Purchase Plan | |||
| Employee benefit plans | |||
| Number of shares authorized for issue | 2,500,000 | ||
| Number of shares that remain available for issuance | 800,000 | ||
| 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 | 362,000 | 283,000 | 268,000 |