Stockholders' Equity and Employee Benefit Plans - Summary of Restricted Stock Units (Details) - Restricted Stock Units - $ / shares	1 Months Ended	12 Months Ended
	Feb. 28, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Number of Shares
Unvested, Beginning balance		8,838,281	8,533,775	10,540,093
Granted	3,820,944	3,911,841	2,936,437	5,492,600
Vested, gross of shares withheld for employee payroll taxes		(5,883,890)	(1,443,302)	(2,546,058)
Forfeited/canceled, including shares withheld for employee payroll taxes		(1,114,186)	(1,188,629)	(4,952,860)
Unvested, Ending balance		5,752,046	8,838,281	8,533,775	10,540,093
Weighted Average Grant-Date Fair Value
Unvested, Beginning balance		$ 12.52	$ 12.89	$ 13.70
Granted		13.26	11.91	12.22
Vested, gross of shares withheld for employee payroll taxes		12.75	13.52	11.58
Forfeited/canceled, including shares withheld for employee payroll taxes		12.65	12.71	13.57
Unvested, Ending balance		$ 12.77	$ 12.52	$ 12.89	$ 13.70
Weighted Average Remaining Contractual Term (in years)
Unvested, Weighted Average Remaining Contractual Term		1 year 2 months 12 days	2 years 2 months 12 days	3 years 3 months 18 days	3 years 6 months