Quarterly report pursuant to Section 13 or 15(d)

Fair Value Measurements - Schedule of financial assets and liabilities (Details)

v3.8.0.1
Fair Value Measurements - Schedule of financial assets and liabilities (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Sep. 30, 2017
Dec. 31, 2016
Sep. 30, 2016
Financial assets and liabilities at carrying value and fair value        
Cash and cash equivalents $ 77.8 $ 66.0 $ 46.6 $ 52.0
Assets:        
Long-term Debt 480.8 465.7    
Level 1        
Assets:        
Cash and cash equivalents 77.8      
Investments in rabbi trust 6.8      
Derivative instruments 0.0      
Liabilities:        
Derivative instruments 0.0      
Level 2        
Assets:        
Cash and cash equivalents 0.0      
Investments in rabbi trust 0.0      
Derivative instruments 4.1      
Liabilities:        
Derivative instruments 1.7      
Level 3        
Assets:        
Cash and cash equivalents 0.0      
Investments in rabbi trust 0.0      
Derivative instruments 0.0      
Liabilities:        
Derivative instruments 0.0      
$150 senior unsecured notes        
Assets:        
Long-term Debt 148.9 148.9    
$150 senior unsecured notes | Level 1        
Liabilities:        
Debt instruments 159.7      
$150 senior unsecured notes | Level 2        
Liabilities:        
Debt instruments 0.0      
$150 senior unsecured notes | Level 3        
Liabilities:        
Debt instruments 0.0      
Revolving credit facility        
Assets:        
Long-term Debt 228.5 68.0    
Revolving credit facility | Level 1        
Liabilities:        
Debt instruments 0.0      
Revolving credit facility | Level 2        
Liabilities:        
Debt instruments 228.5      
Revolving credit facility | Level 3        
Liabilities:        
Debt instruments 0.0      
$100 Series A Notes        
Assets:        
Long-term Debt 99.6 $ 99.7    
$100 Series A Notes | Level 1        
Liabilities:        
Debt instruments 0.0      
$100 Series A Notes | Level 2        
Liabilities:        
Debt instruments 105.4      
$100 Series A Notes | Level 3        
Liabilities:        
Debt instruments 0.0      
Derivatives | Cash flow hedging        
Financial assets and liabilities at carrying value and fair value        
Derivative, notional amount 247.0      
Reported Value Measurement        
Financial assets and liabilities at carrying value and fair value        
Cash and cash equivalents 77.8      
Assets:        
Investments in rabbi trust 6.8      
Derivative instruments 4.1      
Liabilities:        
Derivative instruments $ 1.7