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Stock-Based Compensation (Narrative) (Details)
$ / shares in Units, $ in Thousands
12 Months Ended
Dec. 31, 2017
USD ($)
employee
$ / shares
shares
Dec. 31, 2016
USD ($)
$ / shares
shares
Dec. 31, 2015
USD ($)
$ / shares
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Annual vesting percentage 25.00%    
Vesting period 3 years    
Expiration period 10 years    
Capitalized Amount | $ $ 400 $ 500 $ 600
Allocated stock-based compensation expense | $ $ 11,644 $ 12,286 $ 9,255
Number of options vested and expected to vest (in shares) | shares 1,500 11,500 31,500
Termination      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expiration period 90 days    
Death, Disability, or Retirement      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expiration period 365 days    
Employee Stock Option      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expiration period 10 years    
Aggregate Intrinsic Value | $ $ 226 $ 2,700 $ 3,200 [1]
Allocated stock-based compensation expense | $ 7,700 7,400 5,300
Unrecognized compensation expense related to granted but unvested stock options | $ $ 14,500    
Unrecognized compensation cost, period for recognition 2 years 8 months 12 days    
Number of options vested and expected to vest (in shares) | shares 10,629,743    
Restricted Stock      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Unrecognized compensation expense related to granted but unvested stock options | $ $ 5,200    
Unrecognized compensation cost, period for recognition 1 year 9 months 50 days    
Fair value of shares vested | $ $ 1,800 2,600 2,200
Restricted stock expense | $ $ 3,000 $ 3,700 $ 2,800
Stock units granted (in shares) | shares 1,590,662    
Grants in period, weighted average fair value (usd per share) | $ / shares $ 5.01 $ 8.29 $ 16.03
Shares of unvested restricted stock (in shares) | shares 2,136,443 1,310,019  
Performance stock units      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock units granted (in shares) | shares [2] 513,011    
2015 Stock Incentive Plan      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Common stock, shares authorized (shares) | shares 9,800,000    
Shares available for grant under the plan (in shares) | shares 900,000    
2015 Stock Incentive Plan | Employee Stock Option      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Estimated forfeiture rate 11.00% 11.00% 14.00%
2015 Stock Incentive Plan | Employee Stock Option | Minimum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Option exercise price as a percentage of grant date fair value 100.00%    
2017 Inducement Stock Incentive Plan      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Common stock, shares authorized (shares) | shares 2,000,000    
Employee Stock Purchase Plan      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Shares available for grant under the plan (in shares) | shares 1,000,000    
Discount from market price 15.00%    
Unrecognized compensation expense related to granted but unvested stock options | $ $ 850 $ 1,205 $ 921
Non-Employee Restricted Stock | Restricted Stock      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated stock-based compensation expense | $ $ 79 $ 0 $ 100
Stock units granted (in shares) | shares 0    
Shares of unvested restricted stock (in shares) | shares 41,000 41,000 72,000
Award Modifications | Performance stock units      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated stock-based compensation expense | $ $ 286 $ 273 $ 46
Number of employees affected by award modification | employee 1    
[1] Represents the total difference between the Company's stock price at the time of exercise and the stock option exercise price, multiplied by the number of options exercised.
[2] Shares granted in 2017 include 513,011 performance stock units that have certain performance conditions required to be achieved to vest.