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Summary of Significant Accounting Policies - Stock-Based Compensation (Details)
1 Months Ended 3 Months Ended 6 Months Ended 12 Months Ended
Sep. 30, 2018
installment
shares
Jun. 30, 2020
USD ($)
$ / shares
shares
Dec. 31, 2019
USD ($)
$ / shares
shares
Sep. 30, 2019
USD ($)
Jun. 30, 2019
USD ($)
$ / shares
shares
Mar. 31, 2019
USD ($)
Jun. 30, 2020
USD ($)
plan
installment
$ / shares
shares
Jun. 30, 2019
USD ($)
$ / shares
shares
Dec. 31, 2019
item
$ / shares
shares
Dec. 31, 2017
item
shares
Dec. 31, 2016
item
shares
Jun. 30, 2018
shares
Stock-Based Compensation                        
Number of employee share-based compensation plans | plan             3          
Number of Stock Options                        
Exercised (in shares)             (130,000)          
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]                        
Proceeds from Stock Options Exercised | $             $ 664,000 $ 738,000        
Directors' deferred share unit compensation | $   $ 45,000 $ 91,000 $ 46,000 $ 100,000 $ 100,000            
ESPP                        
Weighted-average assumptions used to estimate the fair value of each stock option                        
Dividend (as a percent)   0.00%     0.00%              
Volatility (as a percent)   85.70%     67.30%              
Risk-free interest rate (as a percent)   1.57%     2.51%              
Expected life   6 months     6 months              
Weighted-Average Grant Date Fair Value                        
Awarded (in dollars per share) | $ / shares             $ 1.86 $ 1.63        
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]                        
Aggregate number of common shares reserved for future issuance                       1,000,000
Allocated Share-based Compensation Expense | $             $ 140,000 $ 292,000        
Shares, Issued   78,000     323,000   78,000 323,000        
Stock options and restricted stock awards                        
Stock-Based Compensation                        
Vesting period             3 years          
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]                        
Allocated Share-based Compensation Expense | $   $ 3,400,000     $ 2,100,000   $ 6,500,000 $ 7,100,000        
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized | $   $ 23,200,000         $ 23,200,000          
Stock options                        
Weighted-average assumptions used to estimate the fair value of each stock option                        
Dividend (as a percent)   0.00%     0.00%   0.00% 0.00%        
Volatility (as a percent)   88.00%     80.30%   84.70% 73.80%        
Risk-free interest rate (as a percent)   0.41%     2.04%   1.30% 2.46%        
Expected life   6 years     6 years   6 years 6 years        
Weighted-average grant date fair value (in dollars per share) | $ / shares   $ 3.39     $ 1.63   $ 3.26 $ 3.39        
Number of Stock Options                        
Outstanding at the beginning of the period (in shares)             13,518,000          
Granted (in shares)             6,686,000          
Exercised (in shares)             (130,000)          
Forfeited/Canceled (in shares)             (1,306,000)          
Outstanding at the end of the period (in shares)   18,768,000 13,518,000       18,768,000   13,518,000      
Weighted-Average Exercise Price                        
Outstanding at the beginning of the period (in dollars per share) | $ / shares             $ 7.53          
Granted (in dollars per share) | $ / shares             4.57          
Exercised (in dollars per share) | $ / shares             2.78          
Forfeited/Canceled (in dollars per share) | $ / shares             9.87          
Outstanding at the end of the period (in dollars per share) | $ / shares   $ 6.35 $ 7.53       $ 6.35   $ 7.53      
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]                        
Proceeds from Stock Options Exercised | $             $ 362,000          
Stock options | Minimum                        
Weighted-Average Exercise Price                        
Exercised (in dollars per share) | $ / shares             $ 2.47          
Stock options | Maximum                        
Weighted-Average Exercise Price                        
Exercised (in dollars per share) | $ / shares             $ 3.05          
Performance shares                        
Stock-Based Compensation                        
Number of equal installments | installment 2           4          
Number of Stock Options                        
Granted (in shares)             2,500,000          
Outstanding at the end of the period (in shares)   139,100         139,100          
Number of Restricted Stock Shares                        
Awarded (in shares) 295,200               57,400 57,400 57,400  
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]                        
Allocated Share-based Compensation Expense | $             $ 0          
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized | $   $ 8,900,000         $ 8,900,000          
Restricted stock                        
Stock-Based Compensation                        
Number of equal installments | item                 3 3 3  
Number of Restricted Stock Shares                        
Unvested at the beginning of the period (in shares)             1,297,000          
Vested (in shares)             (513,000)          
Forfeited (in shares)             (487,000)          
Unvested at the end of the period (in shares)   297,000 1,297,000       297,000   1,297,000      
Weighted-Average Grant Date Fair Value                        
Unvested at the beginning of the period (in dollars per share) | $ / shares             $ 2.97          
Vested (in dollars per share) | $ / shares             2.62          
Forfeited (in dollars per share) | $ / shares             3.62          
Unvested at the end of the period (in dollars per share) | $ / shares   $ 2.55 $ 2.97       $ 2.55   $ 2.97      
Restricted stock | Officers                        
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]                        
Allocated Share-based Compensation Expense | $             $ 0          
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized | $   $ 142,000         $ 142,000          
Previous 2016 and 2006 stock option plans                        
Stock-Based Compensation                        
Common stock authorized for issuance (in shares)   19,500,000         19,500,000          
2018 Plan                        
Stock-Based Compensation                        
Common stock authorized for issuance (in shares)   7,500,000         7,500,000          
Inducement Plan                        
Stock-Based Compensation                        
Number of employee share-based compensation plans | plan             2          
Common stock authorized for issuance (in shares)     1,500,000           1,500,000      
Vesting period             4 years          
Exercise period             10 years