Stock-Based Compensation Expense - Summary of Stock Option Activity (Detail) - Un-vested, Performance-Based Options	6 Months Ended
	Jun. 30, 2018 $ / shares shares
Number of shares
Outstanding at beginning of period (in shares) | shares	11,967,815
Granted (in shares) | shares	980,000
Exercised (in shares) | shares	(204,521)
Forfeited or Expired (in shares) | shares	(1,699,249)
Outstanding at end of period (in shares) | shares	11,044,045
Vested and expected to vest at period end (in shares) | shares	8,917,383
Exercisable at period end (in shares) | shares	5,809,968
Weighted-average exercise price
Outstanding at beginning of period (in dollars per share) | $ / shares	$ 6.73
Granted (in dollars per share) | $ / shares	4.11
Exercised (in dollars per share) | $ / shares	3.80
Forfeited and Expired (in dollars per share) | $ / shares	5.85
Outstanding at end of period (in dollars per share) | $ / shares	6.68
Vested and expected to vest at period end (in dollars per share) | $ / shares	6.93
Exercisable at period end (in dollars per share) | $ / shares	$ 8.00