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) | (14.8) |
Other comprehensive income (loss) | 5 | (31.6) | 34.2 | (59) |
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 |
Amounts Reclassified to Net Income (Loss), Pretax | 0 | 0 | 0 | 0 |
Amounts Reclassified to Net Income (Loss), Income Tax | 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 | (0.3) |
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 | 0.5 | 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 | (0.4) | 0 |
Other comprehensive income (loss) | 0.1 | (0.4) | (0.1) | 0 |
Accumulated Other Comprehensive Loss | | | | |
Other Comprehensive Income Loss [Line Items] | | | | |
Amounts Arising During the Current Period, Pretax | (4) | (36.8) | 6 | (83) |
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 | 38.8 | 40.2 |
Amounts Reclassified to Net Income (Loss), Income Tax | (4.2) | (4.8) | (13) | (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 |
Amounts Arising During the Current Period, Income Tax | 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 |
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 |
Amounts Reclassified to Net Income (Loss), Pretax | 0 | 0 | 0 | 0 |
Amounts Reclassified to Net Income (Loss), Income Tax | 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 |
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 |
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 | 24.6 |
Foreign currency translation adjustments | | | | |
Other Comprehensive Income Loss [Line Items] | | | | |
Amounts Arising During the Current Period, Pretax | (3) | (39.8) | 11.3 | (83.3) |
Amounts Arising During the Current Period, Income Tax | 0 | 0 | 0 | 0 |
Amounts Reclassified to Net Income (Loss), Pretax | 0 | 0 | 0 | 0 |
Amounts Reclassified to Net Income (Loss), Income Tax | 0 | 0 | 0 | 0 |
Other comprehensive income (loss) | (3) | (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 | (0.3) |
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 | 0.5 | 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 | (0.4) | 0 |
Other comprehensive income (loss) | $ 0.1 | $ (0.4) | $ (0.1) | $ 0 |