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Fair value measurements (Details) - USD ($)
$ in Millions
Jun. 30, 2018
Dec. 31, 2017
ASSETS:    
Derivative instruments $ 3.2 $ 1.7
Fair Value, Measurements, Recurring [Member]    
ASSETS:    
Cash equivalents 94.6 142.8
Trust assets 0.9 1.8
Surety bonds 2.1 1.6
Derivative instruments 3.2 1.7
TOTAL 100.8 147.9
LIABILITIES:    
Contingent obligation - net 0.0 0.0
Derivative instruments 1.5 1.2
TOTAL 1.5 1.2
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]    
ASSETS:    
Cash equivalents 94.6 142.8
Trust assets 0.9 1.8
Surety bonds 2.1 1.6
Derivative instruments 0.0 0.0
TOTAL 97.6 146.2
LIABILITIES:    
Contingent obligation - net 0.0 0.0
Derivative instruments 0.0 0.0
TOTAL 0.0 0.0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]    
ASSETS:    
Cash equivalents 0.0 0.0
Trust assets 0.0 0.0
Surety bonds 0.0 0.0
Derivative instruments 0.0 0.0
TOTAL 0.0 0.0
LIABILITIES:    
Contingent obligation - net 0.0 0.0
Derivative instruments 0.0 0.0
TOTAL 0.0 0.0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]    
ASSETS:    
Cash equivalents 0.0 0.0
Trust assets 0.0 0.0
Surety bonds 0.0 0.0
Derivative instruments 3.2 1.7
TOTAL 3.2 1.7
LIABILITIES:    
Contingent obligation - net 0.0 0.0
Derivative instruments 1.5 1.2
TOTAL $ 1.5 $ 1.2