Annual report pursuant to Section 13 and 15(d)

Financing Arrangements - Narrative (Details)

v3.8.0.1
Financing Arrangements - Narrative (Details)
1 Months Ended 12 Months Ended
Aug. 31, 2014
USD ($)
d
$ / shares
shares
Dec. 31, 2017
USD ($)
d
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Debt Instrument [Line Items]        
Carrying value of equity component   $ 18,859,000 $ 29,563,000  
Reclassification of equity component of currently redeemable convertible notes   10,704,000 10,065,000 $ (39,628,000)
Warrants to purchase shares (shares) | shares 3,264,643      
Exercise price of convertible bond hedges (USD per share) | $ / shares $ 75.05      
Exercise price of warrants (USD per share) | $ / shares $ 125.08      
Sale of warrants $ 11,600,000      
2019 Convertible Senior Notes        
Debt Instrument [Line Items]        
Principal amount outstanding   $ 245,000,000 245,000,000  
Senior Notes        
Debt Instrument [Line Items]        
Payments for convertible bond hedges 48,100,000      
Senior Notes | 2019 Convertible Senior Notes        
Debt Instrument [Line Items]        
Principal amount outstanding 245,000,000.0      
Net proceeds from debt issuance $ 239,300,000      
Convertible debt conversion ratio 0.0133251      
Debt conversion price per share | $ / shares $ 75.05      
Interest on debt instrument (as a percent) 0.75%      
Number of consecutive trading days | d   50    
Carrying value of equity component   $ 51,300,000    
Proceeds from issuance of debt $ 245,000,000.0      
Discount rate (as a percent) 5.83%      
Convertible debt, noncurrent $ 192,500,000      
If-converted value of debt in excess of principal   202,000,000    
Debt issuance cost 5,700,000      
Reclassification of equity component of currently redeemable convertible notes 1,200,000      
Unamortized debt issuance costs $ 4,500,000      
Debt issuance costs, net   1,600,000 $ 2,500,000  
Term of debt 5 years      
Working capital deficit   $ 1,800,000    
Threshold business days for repayment of principal   3 days    
Senior Notes | 2019 Convertible Senior Notes | Redemption Period One        
Debt Instrument [Line Items]        
Number of trading days | d 20      
Number of consecutive trading days | d 30      
Stock price trigger threshold (as a percent) 130.00%      
Senior Notes | 2019 Convertible Senior Notes | Redemption Period Two        
Debt Instrument [Line Items]        
Number of trading days | d 5      
Number of consecutive trading days | d 10      
Debt threshold trigger price (as a percent) 98.00%