Income taxes (Tables)
 9 Months Ended
Sep. 30, 2018
Income Tax Disclosure [Abstract]  
Schedule of Components of Income Tax Expense (Benefit)

Three Months Ended September 30,
 
Nine Months Ended September 30,
 
2018
 
2017
 
2018
 
2017
Continuing operations
 
 
 
 
 
 
 
Provision for income taxes (in millions)
$
23.0

 
16.4

 
$
53.0

 
48.1

Effective tax rate
54.9
%
 
43.7
%
 
(514.6
%)
 
39.0
%