Accounting for Shared-based Payments - Schedule of Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Weighted Average Remaining Contractual Term
Stock based compensation expense	$ 22,715	$ 12,575	$ 9,724
Stock Options | $0.44-17.79
Number of Options
Balance outstanding, beginning of period (in shares)		16,567,020
Balance outstanding, end of period (in shares)			16,567,020
Exercise Price Per Share
Exercise price, low end of the range (in dollars per share)				$ 0.44
Exercise price, high end of the range (in dollars per share)				$ 17.79
Weighted Average Exercise Price Per Share
Balance outstanding, beginning of period (in dollars per share)		$ 3.20
Balance outstanding, end of period (in dollars per share)			$ 3.20
Intrinsic Value
Balance outstanding			$ 8,949
Weighted Average Remaining Contractual Term
Balance outstanding		7 years 3 months 15 days
Stock Options | $2.94-9.33
Number of Options
Granted (in shares)		4,961,112
Exercise Price Per Share
Exercise price, low end of the range (in dollars per share)			$ 2.94
Exercise price, high end of the range (in dollars per share)			9.33
Weighted Average Exercise Price Per Share
Granted (in dollars per share)		$ 6.51
Intrinsic Value
Granted		$ 0
Stock Options | $2.98-6.28
Number of Options
Exercised (in shares)		(269,720)
Exercise Price Per Share
Exercise price, low end of the range (in dollars per share)			2.98
Exercise price, high end of the range (in dollars per share)			6.28
Weighted Average Exercise Price Per Share
Exercised (in dollars per share)		$ 4.24
Intrinsic Value
Exercised		$ 904
Stock Options | $2.98-9.45
Number of Options
Forfeited (in shares)		(305,037)
Exercise Price Per Share
Exercise price, low end of the range (in dollars per share)			2.98
Exercise price, high end of the range (in dollars per share)			$ 9.45
Weighted Average Exercise Price Per Share
Forfeited (in dollars per share)		$ 6.22
Intrinsic Value
Forfeited		$ 0
Stock Options | $0.44-9.12
Number of Options
Balance outstanding, beginning of period (in shares)	27,867,171	20,953,375
Balance outstanding, end of period (in shares)	29,868,291	27,867,171	20,953,375
Exercisable at the end of the period (in shares)	21,576,652
Exercise Price Per Share
Exercise price, low end of the range (in dollars per share)	$ 0.44	$ 0.44	$ 0.44
Exercise price, high end of the range (in dollars per share)	9.12	9.12	9.12
Weighted Average Exercise Price Per Share
Balance outstanding, beginning of period (in dollars per share)	3.78	3.86
Balance outstanding, end of period (in dollars per share)	3.83	$ 3.78	$ 3.86
Exercisable at the end of the period (in dollars per share)	$ 4.52
Intrinsic Value
Balance outstanding	$ 5,346	$ 65,618	$ 42,438
Intrinsic value of options exercisable, end of period	$ 5,107
Weighted Average Remaining Contractual Term
Balance outstanding	6 years 1 month 2 days	7 years 18 days	7 years 1 month 24 days
Exercisable at the end of the period	5 years 18 days
Stock Options | $2.93-5.63
Number of Options
Granted (in shares)	7,537,000
Exercise Price Per Share
Exercise price, low end of the range (in dollars per share)		$ 2.93
Exercise price, high end of the range (in dollars per share)		5.63
Weighted Average Exercise Price Per Share
Granted (in dollars per share)	$ 3.99
Intrinsic Value
Granted	$ 0
Stock Options | $2.94-4.61
Number of Options
Exercised (in shares)	(70,185)
Exercise Price Per Share
Exercise price, low end of the range (in dollars per share)		2.94
Exercise price, high end of the range (in dollars per share)		4.61
Weighted Average Exercise Price Per Share
Exercised (in dollars per share)	$ 3.17
Intrinsic Value
Exercised	$ 143
Stock Options | $2.98-9.12
Number of Options
Forfeited (in shares)	(553,019)
Exercise Price Per Share
Exercise price, low end of the range (in dollars per share)		2.98
Exercise price, high end of the range (in dollars per share)		$ 9.12
Weighted Average Exercise Price Per Share
Forfeited (in dollars per share)	$ 5.98
Intrinsic Value
Forfeited	$ 0
Stock Options | $1.89-6.77
Number of Options
Granted (in shares)	3,198,000
Exercise Price Per Share
Exercise price, low end of the range (in dollars per share)	$ 1.89
Exercise price, high end of the range (in dollars per share)	6.77
Weighted Average Exercise Price Per Share
Granted (in dollars per share)	$ 3.95
Intrinsic Value
Granted	$ 0
Stock Options | $0.50-5.34
Number of Options
Exercised (in shares)	(99,978)
Exercise Price Per Share
Exercise price, low end of the range (in dollars per share)	$ 0.50
Exercise price, high end of the range (in dollars per share)	5.34
Weighted Average Exercise Price Per Share
Exercised (in dollars per share)	$ 5.48
Intrinsic Value
Exercised	$ 201
Stock Options | $0.50-7.91
Number of Options
Forfeited (in shares)	(1,096,902)
Exercise Price Per Share
Exercise price, low end of the range (in dollars per share)	$ 0.50
Exercise price, high end of the range (in dollars per share)	7.91
Weighted Average Exercise Price Per Share
Forfeited (in dollars per share)	$ 4.65
Intrinsic Value
Forfeited	$ 0