Annual report pursuant to Section 13 and 15(d)

Debt Instruments - Narrative (Details)

v3.8.0.1
Debt Instruments - Narrative (Details)
$ / shares in Units, shares in Millions
1 Months Ended 12 Months Ended
Dec. 09, 2015
USD ($)
day
$ / shares
$ / unit
shares
Sep. 30, 2016
USD ($)
May 31, 2011
Dec. 31, 2017
USD ($)
$ / shares
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Dec. 29, 2017
USD ($)
Dec. 28, 2017
USD ($)
Debt Instruments                
Share price (in dollars per share) | $ / shares       $ 4.82        
Outstanding amount         $ 90,000,000.0      
Line of credit                
Debt Instruments                
Maximum borrowings available         120,000,000.0   $ 200,000,000.0  
Option to increase the commitment amount             $ 50,000,000.0 $ 20,000,000.0
Borrowings from credit facility   $ 90,000,000.0            
Credit facility       $ 0 90,000,000      
Available borrowing capacity       $ 194,100,000 28,800,000      
Credit facility term remaining       2 years        
Amortization of debt issuance costs       $ 400,000 400,000 $ 200,000    
Line of credit | Minimum                
Debt Instruments                
Interest rate       1.50%        
Line of credit facility, unused capacity, commitment fee percentage     0.25%          
Line of credit | Maximum                
Debt Instruments                
Interest rate       2.00%        
Annual fronting fee percent       0.125%        
Line of credit facility, unused capacity, commitment fee percentage     0.375%          
Line of credit | Alternate base rate | Minimum                
Debt Instruments                
Margin     0.50%          
Line of credit | Alternate base rate | Maximum                
Debt Instruments                
Margin     1.00%          
Line of credit | LIBOR | Minimum                
Debt Instruments                
Margin     1.50%          
Line of credit | LIBOR | Maximum                
Debt Instruments                
Margin     2.00%          
Letter of credit                
Debt Instruments                
Maximum borrowings available       $ 15,000,000.0        
Outstanding amount       1,200,000 $ 1,200,000      
2015 credit facility amendment | Line of credit                
Debt Instruments                
Debt issuance cost       400,000        
2017 credit facility | Line of credit                
Debt Instruments                
Debt issuance cost       $ 1,100,000        
Convertible debt | Notes                
Debt Instruments                
Aggregate principal amount $ 345,000,000.0              
Interest rate 1.75%     1.75%        
Net proceeds from sale of debt $ 336,500,000              
Payments for capped call transactions $ 43,200,000              
Debt instrument, conversion ratio 0.060905              
Debt instrument, conversion price (in dollars per share) | $ / shares $ 16.42              
Debt instrument, redemption price, percentage of principal 100.00%              
Debt conversion, maximum number of shares of common stock convertible by notes (shares) | shares 27.3              
Value of debt $ 233,500,000     $ 301,500,000        
Carrying amount of equity component reported in additional paid-in-capital (103,000,000.0)              
Debt fees and expenses $ 2,600,000              
Initial cap price (in dollars per share) | $ / unit 25.26              
Convertible debt | Notes | Redemption, period 1                
Debt Instruments                
Debt instrument, threshold percentage of stock price trigger 130.00%              
Debt instrument, threshold trading days | day 20              
Debt instrument, threshold consecutive trading days | day 30              
Debt instrument, threshold notice trading days | day 5              
Convertible debt | Notes | Redemption, period 2                
Debt Instruments                
Debt instrument, threshold percentage of stock price trigger 130.00%              
Debt instrument, threshold trading days | day 20              
Debt instrument, threshold consecutive trading days | day 30              
Convertible debt | Notes | Redemption, period 3                
Debt Instruments                
Debt instrument, threshold note trading days | day 5              
Debt instrument, threshold consecutive note trading days 10 days              
Debt instrument, threshold principle amount of note trigger $ 1,000              
Debt instrument, threshold percentage of note price trigger 98.00%