Stock-Based Compensation (Valuation Assumptions) (Details) - $ / shares		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expiration period		10 years
Employee Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Average expected option life (in years)	[1]	5 years 7 months 6 days	5 years 6 months	5 years 6 months
Volatility	[2]	51.30%	44.20%	43.30%
Risk-free interest rate	[3]	1.90%	1.20%	1.60%
Dividend Yield	[4]	0.00%	0.00%	0.00%
Weighted-average grant-date fair value per stock option (in USD per share)		$ 2.51	$ 3.45	$ 6.37
Expiration period		10 years

[1] Determined by the historical stock option exercise behavior of the Company's employees (maximum term is 10 years).

[2] Measured using daily price observations for a period equal to

[3] Based upon the United States Treasury yields in effect (for a period equaling the stock options' expected terms)

[4] The Company has never paid cash dividends on its common stock and does not expect to declare any cash dividends.