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Components of net periodic benefit cost (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Pension Benefits [Member]        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Service cost $ 1.0 $ 1.0 $ 2.1 $ 2.2
Interest cost 3.0 3.3 6.1 6.7
Expected return on plan assets (5.3) (5.1) (10.6) (9.9)
Amortization of prior service costs 0.1 0.0 0.1 0.1
Amortization of net loss 1.1 1.5 2.6 2.7
Net periodic benefit cost (0.1) 0.7 0.3 1.8
Other Postretirement Benefits [Member]        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Service cost 0.0 0.2 0.1 0.4
Interest cost (1.8) 3.0 (0.7) 4.3
Amortization of prior service costs (1.5) (0.7) (3.3) (1.0)
Amortization of net loss 3.3 (1.0) 4.4 0.0
Net periodic benefit cost $ 0.0 $ 1.5 $ 0.5 $ 3.7