Annual report pursuant to Section 13 and 15(d)

Stock Incentive Plan - Stock Award Activity (Details)

v3.8.0.1
Stock Incentive Plan - Stock Award Activity (Details) - USD ($)
$ / shares in Units, $ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Additional option disclosures        
Total future stock compensation expense related to nonvested awards $ 16.8      
Weighted average period of recognition of future stock compensation expense 4 years      
Employee Consultants and Directors Stock Options [Member]        
Stock option activity        
Options, Outstanding at the beginning of the period (in shares) 4,340,997 3,228,027 2,743,818  
Options, Granted (in shares) 1,298,355 1,529,850 946,250  
Options, Forfeited (in shares) (94,777) (75,938) (290,478)  
Options, Exercised (in shares) (568,135) (340,942) (171,563)  
Options, Outstanding at the end of the period (in shares) 4,976,440 4,340,997 3,228,027 2,743,818
Options, Exercisable (in shares) 2,343,796      
Weighted Average Exercise Price, Outstanding at the beginning of the period (in dollars per share) $ 4.92 $ 3.40 $ 3.03  
Weighted Average Exercise Price, Granted (in dollars per share) 19.99 7.67 4.31  
Weighted Average Exercise Price, Forfeited (in dollars per share) (6.29) (3.74) (3.12)  
Weighted Average Exercise Price, Exercised (in dollars per share) (3.43) (3.16) (2.99)  
Weighted Average Exercise Price, Outstanding at the end of the period (in dollars per share) 8.99 $ 4.92 $ 3.40 $ 3.03
Weighted Average Exercise Price, Exercisable (in dollars per share) $ 4.12      
Weighted Average Remaining Contractual Term, Outstanding 6 years 4 months 21 days 5 years 11 months 5 days 5 years 5 months 5 days 5 years 11 months 9 days
Weighted Average Remaining Contractual Term, Exercisable 4 years 3 months 4 days      
Additional option disclosures        
Average fair value of options granted $ 19.99 $ 7.67 $ 4.31  
Intrinsic value of options exercised $ 7.8 $ 1.5    
Intrinsic value of options outstanding 96.1 17.7    
Intrinsic value of options exercisable $ 56.7 $ 12.9    
Stock Options        
Additional option disclosures        
Average fair value of options granted $ 8.40 $ 3.84