v3.8.0.1
Debt 0.75 Senior convertible notes due 2022 Narrative (Details)
12 Months Ended
May 13, 2017
Dec. 31, 2017
USD ($)
d
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Jun. 12, 2017
USD ($)
May 12, 2017
USD ($)
$ / shares
Debt Instrument [Line Items]            
Line of Credit, Current   $ 0        
Non-cash interest expense   9,400,000 $ 100,000 $ 200,000    
Principal amount   400,000,000 0      
Proceeds from issuance of convertible debt, net of issuance costs   389,000,000 $ 0 $ 0    
Line of Credit Facility, Remaining Borrowing Capacity   200,000,000        
0.75% Senior Convertible Notes [Member] | Convertible Notes due 2022 [Member]            
Debt Instrument [Line Items]            
Debt Instrument, Convertible, Conversion Ratio 0.0100918          
Senior Notes [Member] | 0.75% Senior Convertible Notes [Member] | Convertible Notes due 2022 [Member]            
Debt Instrument [Line Items]            
Non-cash interest expense   8,200,000        
Principal amount         $ 50,000,000 $ 350,000,000.0
Debt Instrument, Interest Rate, Stated Percentage           0.75%
Proceeds from issuance of convertible debt, net of issuance costs   389,000,000        
Debt Instrument, Convertible, Conversion Price | $ / shares           $ 99.09
Interest Expense, Debt, Excluding Amortization   1,900,000        
Amortization of Debt Issuance Costs   $ 1,000,000        
Debt Instrument, Interest Rate, Effective Percentage   5.14%        
Debt Instrument, Covenant Repurchase Price, Percentage   100.00%        
Debt Instrument, Convertible, Terms of Conversion Feature, Conversion Amount   $ 1,000        
Debt Instrument, Redemption Price, Percentage   100.00%        
Additional paid-in capital [Member] | Senior Notes [Member] | 0.75% Senior Convertible Notes [Member] | Convertible Notes due 2022 [Member]            
Debt Instrument [Line Items]            
Embedded Derivative, No Longer Bifurcated, Amount Reclassified to Stockholders' Equity   $ 72,600,000        
Debt Instrument Conversion Term One [Member] | Senior Notes [Member] | 0.75% Senior Convertible Notes [Member] | Convertible Notes due 2022 [Member] | Minimum [Member]            
Debt Instrument [Line Items]            
Debt Instrument, Convertible, Threshold Percentage of Stock Price Trigger   130.00%        
Debt Instrument, Convertible, Threshold Consecutive Trading Days | d   20        
Debt Instrument Conversion Term One [Member] | Senior Notes [Member] | 0.75% Senior Convertible Notes [Member] | Convertible Notes due 2022 [Member] | Maximum [Member]            
Debt Instrument [Line Items]            
Debt Instrument, Convertible, Threshold Consecutive Trading Days | d   30        
Debt Instrument Conversion Term Two [Member] | Senior Notes [Member] | 0.75% Senior Convertible Notes [Member] | Convertible Notes due 2022 [Member]            
Debt Instrument [Line Items]            
Debt Instrument, Convertible, Threshold Consecutive Trading Days | d   5        
Debt Instrument Conversion Term Two [Member] | Senior Notes [Member] | 0.75% Senior Convertible Notes [Member] | Convertible Notes due 2022 [Member] | Maximum [Member]            
Debt Instrument [Line Items]            
Debt Instrument, Convertible, Threshold Percentage of Stock Price Trigger   98.00%        
Debt Instrument, Convertible, Threshold Consecutive Trading Days | d   5