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Debt (Tables)
 6 Months Ended
Jun. 30, 2018
Debt [Abstract]  
Schedule of Carrying Values and Estimated Fair Values of Debt Instruments
0.75% Senior Convertible Notes due 2022
(in millions)
June 30, 2018
 
December 31, 2017
0.75% Senior convertible notes due 2022:
 
 
 
Principal amount
$
400.0

 
$
400.0

Unamortized debt discount
(57.8
)
 
(64.4
)
Unamortized debt issuance costs
(7.2
)
 
(8.0
)
Net carrying amount of senior convertible notes
$
335.0

 
$
327.6

Fair value of outstanding notes
$
465.8

 
$
381.3

Amount by which the notes' if-converted value exceeds their principal amount
$
55.4

 
$
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