Stock-Based Compensation (Summary of Liability-Classified Restricted Stock Unit Activity) (Details) - RSUs shares in Thousands	3 Months Ended
	Mar. 31, 2019 $ / shares shares
Number of Shares
Beginning balance (in shares) | shares	8,202
Granted (in shares) | shares	8,659
Vested (in shares) | shares	(2,120)
Forfeited (in shares) | shares	(504)
Ending balance (in shares) | shares	14,237
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	4.29
Forfeited (in dollars per share) | $ / shares	3.85
Ending balance (in dollars per share) | $ / shares	$ 4.69