Employee Benefit Plans - Employee Stock Purchase Plan (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Employee Stock Purchase Plan
Employee benefit plans
Required service period	3 months
Class A common stock
Employee benefit plans
Number of common stock available for future grant under stock incentive plans (in shares)	8,000,000
Class A common stock | Employee Stock Purchase Plan
Employee benefit plans
Number of shares authorized for issue (in shares)	5,000,000.0
Number of common stock available for future grant under stock incentive plans (in shares)	2,500,000
Maximum allowed purchase under the 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	245,000	176,000	227,000