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Long-term Debt (Narrative) (Details)
3 Months Ended 12 Months Ended
May 17, 2018
USD ($)
shares
$ / shares
Jan. 16, 2018
USD ($)
day
shares
$ / shares
Mar. 31, 2019
USD ($)
shares
Mar. 31, 2018
USD ($)
Dec. 31, 2018
USD ($)
shares
Apr. 03, 2019
shares
Apr. 01, 2019
shares
Jan. 31, 2019
Nov. 13, 2018
USD ($)
Aug. 08, 2018
USD ($)
Schedule of Capitalization, Long-term Debt [Line Items]                    
Proceeds from borrowings on senior secured convertible notes (related party)     $ 0 $ 85,000,000            
Issuance of PIK Interest Shares (in shares) | shares     66,722,683   66,154,626          
Debt issuance costs     $ 0 4,315,000            
Amortization of deferred issuance costs     252,000 207,000            
Accretion of debt discount     1,319,000 752,000            
Wells Fargo Bank, N.A. | Letter of credit                    
Schedule of Capitalization, Long-term Debt [Line Items]                    
Letters of credit outstanding     3,500,000              
Convertible senior notes                    
Schedule of Capitalization, Long-term Debt [Line Items]                    
Face value of senior secured convertible notes     204,000,000   $ 204,000,000          
Debt issuance discount     21,403,000   22,723,000          
Amortization of deferred issuance costs     300,000 200,000            
Accretion of debt discount     1,300,000 $ 800,000            
Convertible senior notes | Level 3                    
Schedule of Capitalization, Long-term Debt [Line Items]                    
Fair value of notes     207,200,000              
Convertible senior notes | Initial Notes, due January 16, 2022                    
Schedule of Capitalization, Long-term Debt [Line Items]                    
Face value of senior secured convertible notes     $ 153,500,000   $ 153,500,000          
Stated interest rate     12.00%   6.00%          
Debt issuance discount     $ 18,297,000   $ 19,627,000          
Convertible senior notes | Option Notes, due January 16, 2022                    
Schedule of Capitalization, Long-term Debt [Line Items]                    
Face value of senior secured convertible notes     $ 50,500,000   $ 50,500,000          
Stated interest rate     12.00%   6.00%          
Debt issuance discount     $ 3,106,000   $ 3,096,000          
Convertible senior notes | Starboard Value LP | Initial Notes, due January 16, 2022                    
Schedule of Capitalization, Long-term Debt [Line Items]                    
Face value of senior secured convertible notes   $ 150,000,000.0             $ 2,000,000.0  
Proceeds from borrowings on senior secured convertible notes (related party)   $ 85,000,000                
Converted shares issued (in shares) | shares   2,600,000                
Value of amount of shares of common stock   $ 65,000,000                
Share price (in dollars per share) | $ / shares   $ 24.45                
Debt issuance discount at inception of notes   $ 1,400,000                
Right to increase borrowing   $ 50,000,000                
Number of shares callable by warrants (in shares) | shares   250,000                
Conversion price, percentage premium of trading price   30.00%                
Conversion threshold, consecutive trading days | day   10                
Conversion price (in dollars per share) | $ / shares   $ 31.29                
Stated interest rate   6.00%                
Accrued interest liability     5,100,000              
Price of warrants (in dollars per share) | $ / shares   $ 0.01                
Debt issuance discount   $ 20,100,000                
Debt issuance costs   $ 4,600,000                
Convertible senior notes | Starboard Value LP | Initial Notes, due January 16, 2022 | Subsequent event                    
Schedule of Capitalization, Long-term Debt [Line Items]                    
Number of shares callable by warrants (in shares) | shares           323,448        
Issuance of PIK Interest Shares (in shares) | shares             243,261      
Convertible senior notes | Starboard Value LP | Initial Notes, due January 16, 2022 | Minimum                    
Schedule of Capitalization, Long-term Debt [Line Items]                    
Conversion price (in dollars per share) | $ / shares   $ 28.00                
Stated interest rate               4.00%    
Convertible senior notes | Starboard Value LP | Initial Notes, due January 16, 2022 | Maximum                    
Schedule of Capitalization, Long-term Debt [Line Items]                    
Stated interest rate               12.00%    
Convertible senior notes | Starboard Value LP | Additional Senior Secured Convertible Notes                    
Schedule of Capitalization, Long-term Debt [Line Items]                    
Face value of senior secured convertible notes                   $ 2,000,000.0
Convertible senior notes | Starboard Value LP | Additional Initial Notes                    
Schedule of Capitalization, Long-term Debt [Line Items]                    
Face value of senior secured convertible notes                   1,500,000.0
Convertible senior notes | Starboard Value LP | Initial Notes                    
Schedule of Capitalization, Long-term Debt [Line Items]                    
Face value of senior secured convertible notes         204,000,000.0          
Proceeds from borrowings on senior secured convertible notes (related party)         100,000,000          
Debt instrument, debt covenant, minimum cash balance                 $ 20,000,000.0  
Convertible senior notes | Starboard Value LP | Option Notes, due January 16, 2022                    
Schedule of Capitalization, Long-term Debt [Line Items]                    
Face value of senior secured convertible notes $ 50,000,000                  
Proceeds from borrowings on senior secured convertible notes (related party) $ 15,000,000                  
Converted shares issued (in shares) | shares 1,400,000                  
Value of amount of shares of common stock $ 35,000,000                  
Share price (in dollars per share) | $ / shares $ 21.75                  
Debt issuance discount at inception of notes $ 4,600,000                  
Debt issuance discount 3,100,000                  
Debt issuance costs $ 200,000                  
Convertible senior notes | Starboard Value LP | Additional Option Notes                    
Schedule of Capitalization, Long-term Debt [Line Items]                    
Face value of senior secured convertible notes                   $ 500,000.0
Convertible senior notes | Starboard Value LP | Right Offering Notes                    
Schedule of Capitalization, Long-term Debt [Line Items]                    
Maximum percentage exchangeable for common stock   30.00%                
Debt term   4 years                
Conversion price percentage   130.00%                
Backstop commitment by third party   $ 100,000,000                
Backstop reduced by the amount of option notes purchased   50,000,000.0                
Convertible senior notes | Starboard Value LP | Right Offering Notes | Minimum                    
Schedule of Capitalization, Long-term Debt [Line Items]                    
Right to increase borrowing   $ 35,000,000.0                
Conversion price (in dollars per share) | $ / shares   $ 28.00                
Convertible senior notes | Starboard Value LP | Right Offering Notes | Maximum                    
Schedule of Capitalization, Long-term Debt [Line Items]                    
Additional borrowing option   $ 150,000,000.0                
Debt convenant, period one | Convertible senior notes | Starboard Value LP | Initial Notes                    
Schedule of Capitalization, Long-term Debt [Line Items]                    
Debt instrument, debt covenant, minimum cash balance     20,000,000   20,000,000.0          
Debt convenant, period two | Convertible senior notes | Starboard Value LP | Initial Notes                    
Schedule of Capitalization, Long-term Debt [Line Items]                    
Debt instrument, debt covenant, minimum cash balance     $ 40,000,000.0   $ 40,000,000.0