Accumulated other comprehensive (loss) - Comprehensive Income (Loss) (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2016
Sep. 30, 2015
Other Comprehensive Income Loss [Line Items]        
Amounts Arising During the Current Period, Pretax $ (3.6) $ (39.3) $ 7.1 $ (86.6)
Amounts Arising During the Current Period, Income Tax 0.1 0.3 0.9 1.8
Amounts Reclassified to Net Income (Loss), Pretax 12.7 12.2 39.2 40.6
Amounts Reclassified to Net Income (Loss), Income Tax (4.2) (4.8) (13.0) (14.8)
Other comprehensive income (loss) 5.0 (31.6) 34.2 (59.0)
Benefit plan adjustments, Attributable to parent        
Other Comprehensive Income Loss [Line Items]        
Amounts Arising During the Current Period, Pretax (0.7) (1.1) (2.6) (6.8)
Amounts Arising During the Current Period, Income Tax 0.1 0.2 0.6 1.7
Amounts Reclassified to Net Income (Loss), Pretax 12.5 14.4 37.4 44.2
Amounts Reclassified to Net Income (Loss), Income Tax (4.1) (4.9) (12.8) (14.9)
Other comprehensive income (loss) 7.8 8.6 22.6 24.2
Foreign currency translation adjustments, Attributable to parent        
Other Comprehensive Income Loss [Line Items]        
Amounts Arising During the Current Period, Pretax (3.4) (37.3) 10.2 (79.7)
Amounts Arising During the Current Period, Income Tax 0.0 0.0 0.0 0.0
Amounts Reclassified to Net Income (Loss), Pretax 0.0 0.0 0.0 0.0
Amounts Reclassified to Net Income (Loss), Income Tax 0.0 0.0 0.0 0.0
Other comprehensive income (loss) (3.4) (37.3) 10.2 (79.7)
Unrealized gains (losses) on available-for-sale securities, Attributable to parent        
Other Comprehensive Income Loss [Line Items]        
Amounts Arising During the Current Period, Pretax   (0.2) 0.5 (0.2)
Amounts Arising During the Current Period, Income Tax   0.1 (0.2) 0.1
Amounts Reclassified to Net Income (Loss), Pretax   (0.2) (0.5) (0.3)
Amounts Reclassified to Net Income (Loss), Income Tax   0.1 0.2 0.1
Other comprehensive income (loss)   (0.2) 0.0 (0.3)
Gains (losses) on cash flow hedges, Attributable to parent        
Other Comprehensive Income Loss [Line Items]        
Amounts Arising During the Current Period, Pretax 0.1 1.8 (2.1) 3.7
Amounts Arising During the Current Period, Income Tax 0.0 0.0 0.5 0.0
Amounts Reclassified to Net Income (Loss), Pretax 0.1 (2.2) 1.9 (3.7)
Amounts Reclassified to Net Income (Loss), Income Tax (0.1) 0.0 (0.4) 0.0
Other comprehensive income (loss) 0.1 (0.4) (0.1) 0.0
Accumulated Other Comprehensive Loss        
Other Comprehensive Income Loss [Line Items]        
Amounts Arising During the Current Period, Pretax (4.0) (36.8) 6.0 (83.0)
Amounts Arising During the Current Period, Income Tax 0.1 0.3 0.9 1.8
Amounts Reclassified to Net Income (Loss), Pretax 12.6 12.0 38.8 40.2
Amounts Reclassified to Net Income (Loss), Income Tax (4.2) (4.8) (13.0) (14.8)
Other comprehensive income (loss) 4.5 (29.3) 32.7 (55.8)
Benefit plan adjustments, Attributable to noncontrolling interest        
Other Comprehensive Income Loss [Line Items]        
Amounts Arising During the Current Period, Pretax 0.0 0.0 0.0 0.0
Amounts Arising During the Current Period, Income Tax 0.0 0.0 0.0 0.0
Amounts Reclassified to Net Income (Loss), Pretax 0.1 0.2 0.4 0.4
Amounts Reclassified to Net Income (Loss), Income Tax 0.0 0.0 0.0 0.0
Other comprehensive income (loss) 0.1 0.2 0.4 0.4
Foreign currency translation adjustments, Attributable to noncontrolling interest        
Other Comprehensive Income Loss [Line Items]        
Amounts Arising During the Current Period, Pretax 0.4 (2.5) 1.1 (3.6)
Amounts Arising During the Current Period, Income Tax 0.0 0.0 0.0 0.0
Amounts Reclassified to Net Income (Loss), Pretax 0.0 0.0 0.0 0.0
Amounts Reclassified to Net Income (Loss), Income Tax 0.0 0.0 0.0 0.0
Other comprehensive income (loss) 0.4 (2.5) 1.1 (3.6)
AOCI Attributable to Noncontrolling Interest [Member]        
Other Comprehensive Income Loss [Line Items]        
Amounts Arising During the Current Period, Pretax 0.4 (2.5) 1.1 (3.6)
Amounts Arising During the Current Period, Income Tax 0.0 0.0 0.0 0.0
Amounts Reclassified to Net Income (Loss), Pretax 0.1 0.2 0.4 0.4
Amounts Reclassified to Net Income (Loss), Income Tax 0.0 0.0 0.0 0.0
Other comprehensive income (loss) 0.5 (2.3) 1.5 (3.2)
Benefit plan adjustments        
Other Comprehensive Income Loss [Line Items]        
Amounts Arising During the Current Period, Pretax (0.7) (1.1) (2.6) (6.8)
Amounts Arising During the Current Period, Income Tax 0.1 0.2 0.6 1.7
Amounts Reclassified to Net Income (Loss), Pretax 12.6 14.6 37.8 44.6
Amounts Reclassified to Net Income (Loss), Income Tax (4.1) (4.9) (12.8) (14.9)
Other comprehensive income (loss) 7.9 8.8 23.0 24.6
Foreign currency translation adjustments        
Other Comprehensive Income Loss [Line Items]        
Amounts Arising During the Current Period, Pretax (3.0) (39.8) 11.3 (83.3)
Amounts Arising During the Current Period, Income Tax 0.0 0.0 0.0 0.0
Amounts Reclassified to Net Income (Loss), Pretax 0.0 0.0 0.0 0.0
Amounts Reclassified to Net Income (Loss), Income Tax 0.0 0.0 0.0 0.0
Other comprehensive income (loss) (3.0) (39.8) 11.3 (83.3)
Unrealized gains (losses) on available-for-sale securities        
Other Comprehensive Income Loss [Line Items]        
Amounts Arising During the Current Period, Pretax   (0.2) 0.5 (0.2)
Amounts Arising During the Current Period, Income Tax   0.1 (0.2) 0.1
Amounts Reclassified to Net Income (Loss), Pretax   (0.2) (0.5) (0.3)
Amounts Reclassified to Net Income (Loss), Income Tax   0.1 0.2 0.1
Other comprehensive income (loss)   (0.2) 0.0 (0.3)
Gains (losses) on cash flow hedges        
Other Comprehensive Income Loss [Line Items]        
Amounts Arising During the Current Period, Pretax 0.1 1.8 (2.1) 3.7
Amounts Arising During the Current Period, Income Tax 0.0 0.0 0.5 0.0
Amounts Reclassified to Net Income (Loss), Pretax 0.1 (2.2) 1.9 (3.7)
Amounts Reclassified to Net Income (Loss), Income Tax (0.1) 0.0 (0.4) 0.0
Other comprehensive income (loss) $ 0.1 $ (0.4) $ (0.1) $ 0.0