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Fair value measurements (Details) - USD ($)
$ in Millions
Mar. 31, 2018
Dec. 31, 2017
ASSETS:    
Derivative instruments $ 2.0 $ 1.7
Fair Value, Measurements, Recurring [Member]    
ASSETS:    
Cash equivalents 80.0 142.8
Trust assets 1.3 1.8
Surety bonds 1.6 1.6
Derivative instruments 2.0 1.7
TOTAL 84.9 147.9
LIABILITIES:    
Contingent obligation - net 0.0 0.0
Derivative instruments 1.1 1.2
TOTAL 1.1 1.2
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]    
ASSETS:    
Cash equivalents 80.0 142.8
Trust assets 1.3 1.8
Surety bonds 1.6 1.6
Derivative instruments 0.0 0.0
TOTAL 82.9 146.2
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 2.0 1.7
TOTAL 2.0 1.7
LIABILITIES:    
Contingent obligation - net 0.0 0.0
Derivative instruments 1.1 1.2
TOTAL $ 1.1 $ 1.2