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Stock-Based Compensation (Narrative) (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Aggregate intrinsic value of stock options exercised $ 0.4 $ 0.1 $ 2.4
Unrecognized compensation expense $ 45.0    
Unrecognized compensation expense, recognition period 1 year 7 months 6 days    
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Dividend Rate 0.00% 0.00% 0.00%
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value $ 1.74 $ 1.06 $ 1.88
Performance Based Stock Options [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross     1,700,000
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Volatility Rate     99.11%
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Risk Free Interest Rate     1.74%
Sharebased Compensation Arrangement By Sharebased Payment Award Fair Value Assumptions Expected Forfeiture Rate     5.62%
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Dividend Rate     0.00%
Maximum [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Sharebased Compensation Arrangement By Sharebased Payment Award Fair Value Assumptions Expected Forfeiture Rate [1]     16.33%
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Term 7 years 6 months 7 years 5 months 16 days 7 years 3 months 11 days
Maximum [Member] | Performance Based Stock Options [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value     $ 0.92
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Term 3 years 6 months    
Minimum [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Sharebased Compensation Arrangement By Sharebased Payment Award Fair Value Assumptions Expected Forfeiture Rate [1]     0.00%
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Term 4 years 25 days 4 years 1 month 20 days 4 years 2 months 19 days
Minimum [Member] | Performance Based Stock Options [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value     $ 0.74
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Term 1 year 4 months 6 days    
2005 Plan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Maximum term of options 10 years    
Minimum grant price, percent of common stock fair value 100.00%    
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross 0    
Employee Stock Purchase Plan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Common stock reserved for issuance 3,363,066    
Proposal to increase authorized shares 4,000,000    
Employee Stock Purchase Plan (ESPP), Plan Description The ESPP was approved at the Company annual meeting of stockholders in June 2013. The ESPP currently authorizes an aggregate of 7,600,000 shares of the Company's common stock to be purchased, and the aggregate amount of shares will continue to increase 5% on each anniversary of its adoption up to a maximum of 8,000,000 shares. The number of authorized shares and the maximum number of shares both include an increase of 4,000,000 shares approved at the Company's 2018 annual meeting of stockholders. The ESPP allows employees to purchase shares of common stock of the Company at each purchase date through payroll deductions of up to a maximum of 15% of their compensation, at 85% of the lesser of the market price of the shares at the time of purchase or the market price on the beginning date of an option period (or, if later, the date during the option period when the employee was first eligible to participate).    
Employee Stock Purchase Plan [Member] | Maximum [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Common stock reserved for issuance 8,000,000    
2015 Plan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Common stock reserved for issuance 56,000,000    
Aggregate intrinsic value of stock options outstanding $ 10.2    
Weighted-average remaining contractual term of stock options outstanding 7 years 6 months    
Aggregate intrinsic value of stock options exercisable $ 4.3    
Weighted-average remaining contractual term of stock options exercisable 5 years 8 months 12 days    
Proposal to increase authorized shares 20,000,000    
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross 16,613,409    
[1] See Note 3 regarding the Company’s adoption of ASU 2016-09 in 2017.