Stock-Based Compensation (Summary of Liability-Classified Restricted Stock Unit Activity) (Details) - RSUs shares in Thousands	6 Months Ended
	Jun. 30, 2019 $ / shares shares
Number of Shares
Beginning balance (in shares) | shares	8,202
Granted (in shares) | shares	8,659
Vested (in shares) | shares	(2,617)
Forfeited (in shares) | shares	(739)
Ending balance (in shares) | shares	13,505
Weighted Average Fair Value
Beginning balance (in dollars per share) | $ / shares	$ 3.41
Granted (in dollars per share) | $ / shares	4.34
Vested (in dollars per share) | $ / shares	4.09
Forfeited (in dollars per share) | $ / shares	3.13
Ending balance (in dollars per share) | $ / shares	$ 3.16