Stock-Based Compensation (Summary of Liability-Classified Performance Unit Activity) (Details) - Performance units shares in Thousands	3 Months Ended
	Mar. 31, 2019 $ / shares shares
Number of Shares
Beginning balance (in shares) | shares	2,803
Granted (in shares) | shares	2,757
Vested (in shares) | shares	0
Forfeited (in shares) | shares	(119)
Ending balance (in shares) | shares	5,441
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	0
Forfeited (in dollars per share) | $ / shares	4.65
Ending balance (in dollars per share) | $ / shares	$ 4.69