Stock-based Payments - Summary of Stock Option Activity (Details) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 $ / shares
Number of Stock Options
Outstanding, Beginning Balance (in shares)	1,714,593,000
Granted (in shares)	1,391,125,000
Exercised (in shares)	(70,820,000)
Forfeited/cancelled (in shares)	(1,009,483,000)
Outstanding, Ending Balance (in shares)	2,025,415,000	1,714,593,000
Weighted Average Exercised Price
Outstanding, Beginning Balance (in dollars per share) | $ / shares	$ 9.01
Granted (in dollars per share) | (per share)	7.70	$ 8.27	$ 4.84
Exercised (in dollars per share) | $ / shares	7.15
Forfeited/cancelled (in dollars per share) | $ / shares	9.48
Outstanding, Ending Balance (in dollars per share) | $ / shares	$ 8.00	$ 9.01
Options Vested and Expected to Vest
Outstanding, weighted average remaining contractual life (years)	7 years 6 months 11 days	9 years 7 months 24 days
Outstanding, aggregate intrinsic value | $	$ 0	$ 528
Vested and expected to vest at end of year (in shares)	2,025,415,000
Exercisable at end of year (in shares)	430,851,000
Vested and expected to vest at end of year, weighted-average exercise price per share (in dollars per share) | $ / shares	$ 8.00
Exercisable at end of year, weighted-average exercise price per share (in dollars per share) | $ / shares	$ 8.89
Vested and expected to vest at end of year, weighted average remaining contractual life (years)	7 years 6 months 11 days
Exercisable at end of year, weighted average remaining contractual life (years)	3 years 3 months
Vested and expected to vest at end of year, aggregate intrinsic value | $	$ 0
Exercisable at end of year, aggregate intrinsic value | $	$ 0