Share-Based Compensation - Stock Option Activity (Details) - $ / shares	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Shares
Outstanding at beginning of period (in shares)	17,742,966
Granted (in shares)	147,959
Exercised (in shares)	(100,012)
Canceled/forfeited (in shares)	(81,773)
Outstanding at end of period (in shares)	17,709,140
Outstanding, vested and exercisable at end of period (in shares)	5,971,110	5,778,376
Weighted- Average Exercise Price
Outstanding at beginning of period (in dollars per share)	$ 4.70
Granted (in dollars per share)	4.68
Exercised (in dollars per share)	2.43
Canceled/forfeited (in dollars per share)	3.82
Outstanding at end of period (in dollars per share)	4.71
Outstanding, vested and exercisable at end of period (in dollars per share)	$ 8.66	$ 8.87