Accumulated other comprehensive income (loss) - Comprehensive Income (Loss) (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended |
Jun. 30, 2016 | Jun. 30, 2015 | Jun. 30, 2016 | Jun. 30, 2015 |
Other Comprehensive Income Loss [Line Items] | | | | |
Amounts Arising During the Current Period, Pretax | $ (5.2) | $ 5.6 | $ 10.7 | $ (47.3) |
Amounts Arising During the Current Period, Income Tax | 0.4 | 0.3 | 0.8 | 1.5 |
Amounts Reclassified to Net Income (Loss), Pretax | 13.4 | 14.7 | 26.5 | 28.4 |
Amounts Reclassified to Net Income (Loss), Income Tax | (4.6) | (4.8) | (8.8) | (10) |
Other comprehensive income (loss) | 4 | 15.8 | 29.2 | (27.4) |
Benefit plan adjustments, Attributable to parent | | | | |
Other Comprehensive Income Loss [Line Items] | | | | |
Amounts Arising During the Current Period, Pretax | (0.8) | (1.1) | (1.9) | (5.7) |
Amounts Arising During the Current Period, Income Tax | 0.2 | 0.3 | 0.5 | 1.5 |
Amounts Reclassified to Net Income (Loss), Pretax | 12.6 | 14.1 | 24.9 | 29.8 |
Amounts Reclassified to Net Income (Loss), Income Tax | (4.5) | (4.8) | (8.7) | (10) |
Other comprehensive income (loss) | 7.5 | 8.5 | 14.8 | 15.6 |
Foreign currency translation adjustments, Attributable to parent | | | | |
Other Comprehensive Income Loss [Line Items] | | | | |
Amounts Arising During the Current Period, Pretax | (3.5) | 6.5 | 13.6 | (42.4) |
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) | (3.5) | 6.5 | 13.6 | (42.4) |
Unrealized gains (losses) on available-for-sale securities, Attributable to parent | | | | |
Other Comprehensive Income Loss [Line Items] | | | | |
Amounts Arising During the Current Period, Pretax | 0.3 | 0 | 0.5 | 0 |
Amounts Arising During the Current Period, Income Tax | (0.2) | 0 | (0.2) | 0 |
Amounts Reclassified to Net Income (Loss), Pretax | (0.5) | (0.1) | (0.5) | (0.1) |
Amounts Reclassified to Net Income (Loss), Income Tax | 0.2 | 0 | 0.2 | 0 |
Other comprehensive income (loss) | (0.2) | (0.1) | 0 | (0.1) |
Gains (losses) on cash flow hedges, Attributable to parent | | | | |
Other Comprehensive Income Loss [Line Items] | | | | |
Amounts Arising During the Current Period, Pretax | (1.2) | (0.2) | (2.2) | 1.9 |
Amounts Arising During the Current Period, Income Tax | 0.4 | 0 | 0.5 | 0 |
Amounts Reclassified to Net Income (Loss), Pretax | 1.1 | 0.6 | 1.8 | (1.5) |
Amounts Reclassified to Net Income (Loss), Income Tax | (0.3) | 0 | (0.3) | 0 |
Other comprehensive income (loss) | 0 | 0.4 | (0.2) | 0.4 |
Accumulated Other Comprehensive Loss | | | | |
Other Comprehensive Income Loss [Line Items] | | | | |
Amounts Arising During the Current Period, Pretax | (5.2) | 5.2 | 10 | (46.2) |
Amounts Arising During the Current Period, Income Tax | 0.4 | 0.3 | 0.8 | 1.5 |
Amounts Reclassified to Net Income (Loss), Pretax | 13.2 | 14.6 | 26.2 | 28.2 |
Amounts Reclassified to Net Income (Loss), Income Tax | (4.6) | (4.8) | (8.8) | (10) |
Other comprehensive income (loss) | 3.8 | 15.3 | 28.2 | (26.5) |
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 | 0.2 | 0.1 | 0.3 | 0.2 |
Amounts Reclassified to Net Income (Loss), Income Tax | 0 | 0 | 0 | 0 |
Other comprehensive income (loss) | 0.2 | 0.1 | 0.3 | 0.2 |
Foreign currency translation adjustments, Attributable to noncontrolling interest | | | | |
Other Comprehensive Income Loss [Line Items] | | | | |
Amounts Arising During the Current Period, Pretax | 0 | 0.4 | 0.7 | (1.1) |
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 | 0.4 | 0.7 | (1.1) |
AOCI Attributable to Noncontrolling Interest [Member] | | | | |
Other Comprehensive Income Loss [Line Items] | | | | |
Amounts Arising During the Current Period, Pretax | 0 | 0.4 | 0.7 | (1.1) |
Amounts Arising During the Current Period, Income Tax | 0 | 0 | 0 | 0 |
Amounts Reclassified to Net Income (Loss), Pretax | 0.2 | 0.1 | 0.3 | 0.2 |
Amounts Reclassified to Net Income (Loss), Income Tax | 0 | 0 | 0 | 0 |
Other comprehensive income (loss) | 0.2 | 0.5 | 1 | (0.9) |
Benefit plan adjustments | | | | |
Other Comprehensive Income Loss [Line Items] | | | | |
Amounts Arising During the Current Period, Pretax | (0.8) | (1.1) | (1.9) | (5.7) |
Amounts Arising During the Current Period, Income Tax | 0.2 | 0.3 | 0.5 | 1.5 |
Amounts Reclassified to Net Income (Loss), Pretax | 12.8 | 14.2 | 25.2 | 30 |
Amounts Reclassified to Net Income (Loss), Income Tax | (4.5) | (4.8) | (8.7) | (10) |
Other comprehensive income (loss) | 7.7 | 8.6 | 15.1 | 15.8 |
Foreign currency translation adjustments | | | | |
Other Comprehensive Income Loss [Line Items] | | | | |
Amounts Arising During the Current Period, Pretax | (3.5) | 6.9 | 14.3 | (43.5) |
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) | (3.5) | 6.9 | 14.3 | (43.5) |
Unrealized gains (losses) on available-for-sale securities | | | | |
Other Comprehensive Income Loss [Line Items] | | | | |
Amounts Arising During the Current Period, Pretax | 0.3 | 0 | 0.5 | 0 |
Amounts Arising During the Current Period, Income Tax | (0.2) | 0 | (0.2) | 0 |
Amounts Reclassified to Net Income (Loss), Pretax | (0.5) | (0.1) | (0.5) | (0.1) |
Amounts Reclassified to Net Income (Loss), Income Tax | 0.2 | 0 | 0.2 | 0 |
Other comprehensive income (loss) | (0.2) | (0.1) | 0 | (0.1) |
Gains (losses) on cash flow hedges | | | | |
Other Comprehensive Income Loss [Line Items] | | | | |
Amounts Arising During the Current Period, Pretax | (1.2) | (0.2) | (2.2) | 1.9 |
Amounts Arising During the Current Period, Income Tax | 0.4 | 0 | 0.5 | 0 |
Amounts Reclassified to Net Income (Loss), Pretax | 1.1 | 0.6 | 1.8 | (1.5) |
Amounts Reclassified to Net Income (Loss), Income Tax | (0.3) | 0 | (0.3) | 0 |
Other comprehensive income (loss) | $ 0 | $ 0.4 | $ (0.2) | $ 0.4 |