v3.8.0.1
Debt (Tables)
 12 Months Ended
Dec. 31, 2017
Debt Disclosure [Abstract]  
Schedule of Carrying Values and Estimated Fair Values of Debt Instruments [Table Text Block]
0.75% Senior Convertible Notes due 2022
(in millions)
December 31, 2017
 
December 31, 2016
0.75% Senior convertible notes due 2022:
 
 
 
Principal amount
$
400.0

 
$
—

Unamortized debt discount
(64.4
)
 
—

Unamortized debt issuance costs
(8.0
)
 
—

Net carrying amount of senior convertible notes
$
327.6

 
$
—

Fair value of outstanding notes
$
381.3

 
$
—