Accumulated Other Comprehensive Income - Reclasses Out Of AOCI (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Other Comprehensive Income (Loss), Net of Tax [Abstract]    
Beginning balance $ 354.8 $ 330.6
Other comprehensive income 34.3 25.2
Ending balance 421.2 350.2
Benefit Plan Adjustments    
Other Comprehensive Income (Loss), Net of Tax [Abstract]    
Beginning balance (559.6)  
Other comprehensive income (loss) before reclassifications (1.3)  
Amounts reclassified from accumulated other comprehensive loss 8.1  
Other comprehensive income 6.8 7.3
Ending balance (552.8)  
Foreign Currency Translation Adjustments    
Other Comprehensive Income (Loss), Net of Tax [Abstract]    
Beginning balance (349.1)  
Other comprehensive income (loss) before reclassifications 26.3  
Amounts reclassified from accumulated other comprehensive loss 0.0  
Other comprehensive income 26.3 17.1
Ending balance (322.8)  
Unrealized Gains (Losses) on Available-for-Sale Securities    
Other Comprehensive Income (Loss), Net of Tax [Abstract]    
Beginning balance 1.0  
Other comprehensive income (loss) before reclassifications 0.1  
Amounts reclassified from accumulated other comprehensive loss 0.0  
Other comprehensive income 0.1 0.2
Ending balance 1.1  
Gains (Losses) on Cash Flow Hedges    
Other Comprehensive Income (Loss), Net of Tax [Abstract]    
Beginning balance 0.7  
Other comprehensive income (loss) before reclassifications (0.2)  
Amounts reclassified from accumulated other comprehensive loss 0.2  
Other comprehensive income 0.0 (0.2)
Ending balance 0.7  
Accumulated Other Comprehensive Loss    
Other Comprehensive Income (Loss), Net of Tax [Abstract]    
Beginning balance (907.0) (891.9)
Other comprehensive income (loss) before reclassifications 24.9  
Amounts reclassified from accumulated other comprehensive loss 8.3  
Other comprehensive income 33.2 24.4
Ending balance $ (873.8) $ (867.5)