Accumulated other comprehensive income (loss) - Amounts in OCI (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended |
Jun. 30, 2019 | Mar. 31, 2019 | Jun. 30, 2018 | Mar. 31, 2018 | Jun. 30, 2019 | Jun. 30, 2018 |
Other Comprehensive Income Loss [Line Items] | | | | | | |
Amounts Arising During the Current Period, Pretax | $ (7.4) | | $ (137.5) | | $ (13.4) | $ (137.1) |
Amounts Arising During the Current Period, Income Tax | 4.3 | | 0.1 | | 5.6 | 0.3 |
Amounts Reclassified to Net Income (Loss), Pretax | 13.8 | | 129.9 | | 22.5 | 144.9 |
Amounts Reclassified to Net Income (Loss), Income Tax | (3.6) | | (3.9) | | (5.4) | (7.3) |
Other comprehensive income (loss) | 7.1 | $ 2.2 | (11.4) | $ 12.2 | 9.3 | 0.8 |
Cost of revenues | 708.5 | | 666.8 | | 1,411.2 | 1,360.4 |
Selling, general and administrative expenses | 154.6 | | 119.9 | | 296.3 | 243 |
Interest and other income (expense) | (3.1) | | (8.1) | | (14.3) | (21.2) |
Other operating income (expense) | (1.7) | | 1.3 | | 0.5 | (1.1) |
Interest expense | 22.7 | | 15.8 | | 45.7 | 30.8 |
Benefit plan adjustments | | | | | | |
Other Comprehensive Income Loss [Line Items] | | | | | | |
Amounts Arising During the Current Period, Pretax | (2.3) | | 1.3 | | (3.6) | 0.3 |
Amounts Arising During the Current Period, Income Tax | 0.4 | | 0.2 | | 0.6 | 0.5 |
Amounts Reclassified to Net Income (Loss), Pretax | 10.2 | | 22.9 | | 21.5 | 37.7 |
Amounts Reclassified to Net Income (Loss), Income Tax | (2.4) | | (3.4) | | (5.1) | (6.8) |
Other comprehensive income (loss) | 5.9 | | 21 | | 13.4 | 31.7 |
Foreign currency translation adjustments | | | | | | |
Other Comprehensive Income Loss [Line Items] | | | | | | |
Amounts Arising During the Current Period, Pretax | 9 | | (138.2) | | 9.3 | (138.1) |
Amounts Arising During the Current Period, Income Tax | 0 | | 0 | | 0 | 0 |
Amounts Reclassified to Net Income (Loss), Pretax | 0 | | 107.2 | | 0 | 107.2 |
Amounts Reclassified to Net Income (Loss), Income Tax | 0 | | (0.5) | | 0 | (0.5) |
Other comprehensive income (loss) | 9 | | (31.5) | | 9.3 | (31.4) |
Gains (losses) on cash flow hedges | | | | | | |
Other Comprehensive Income Loss [Line Items] | | | | | | |
Amounts Arising During the Current Period, Pretax | (14.2) | | 0.2 | | (19.5) | 0.6 |
Amounts Arising During the Current Period, Income Tax | 3.9 | | (0.1) | | 5 | (0.2) |
Amounts Reclassified to Net Income (Loss), Pretax | 3.6 | | 0 | | 1 | 0 |
Amounts Reclassified to Net Income (Loss), Income Tax | (1.2) | | 0 | | (0.3) | 0 |
Other comprehensive income (loss) | (7.9) | | 0.1 | | (13.8) | 0.4 |
Gains (losses) on cash flow hedges | Reclassification out of accumulated other comprehensive income | | | | | | |
Other Comprehensive Income Loss [Line Items] | | | | | | |
Other operating income (expense) | 2.4 | | 0 | | (1.4) | 0 |
Interest expense | 1.3 | | | | 2.5 | |
AOCI Attributable to Parent | | | | | | |
Other Comprehensive Income Loss [Line Items] | | | | | | |
Amounts Arising During the Current Period, Pretax | (7.5) | | (136.7) | | (13.8) | (137.2) |
Amounts Arising During the Current Period, Income Tax | 4.3 | | 0.1 | | 5.6 | 0.3 |
Amounts Reclassified to Net Income (Loss), Pretax | 13.8 | | 130.1 | | 22.5 | 144.9 |
Amounts Reclassified to Net Income (Loss), Income Tax | (3.6) | | (3.9) | | (5.4) | (7.3) |
Other comprehensive income (loss) | 7 | $ 1.9 | (10.4) | $ 11.1 | 8.9 | 0.7 |
Benefit plan adjustments | | | | | | |
Other Comprehensive Income Loss [Line Items] | | | | | | |
Amounts Arising During the Current Period, Pretax | | | 0 | | | |
Amounts Arising During the Current Period, Income Tax | | | 0 | | | |
Amounts Reclassified to Net Income (Loss), Pretax | | | (0.2) | | | |
Amounts Reclassified to Net Income (Loss), Income Tax | | | 0 | | | |
Other comprehensive income (loss) | | | (0.2) | | | |
Foreign currency translation adjustments | | | | | | |
Other Comprehensive Income Loss [Line Items] | | | | | | |
Amounts Arising During the Current Period, Pretax | 0.1 | | (0.8) | | 0.4 | 0.1 |
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) | 0.1 | | (0.8) | | 0.4 | 0.1 |
AOCI Attributable to Noncontrolling Interest | | | | | | |
Other Comprehensive Income Loss [Line Items] | | | | | | |
Amounts Arising During the Current Period, Pretax | 0.1 | | (0.8) | | 0.4 | 0.1 |
Amounts Arising During the Current Period, Income Tax | 0 | | 0 | | 0 | 0 |
Amounts Reclassified to Net Income (Loss), Pretax | 0 | | (0.2) | | 0 | 0 |
Amounts Reclassified to Net Income (Loss), Income Tax | 0 | | 0 | | 0 | 0 |
Other comprehensive income (loss) | 0.1 | | (1) | | 0.4 | 0.1 |
Benefit plan adjustments(a) | | | | | | |
Other Comprehensive Income Loss [Line Items] | | | | | | |
Amounts Arising During the Current Period, Pretax | (2.3) | | 1.3 | | (3.6) | 0.3 |
Amounts Arising During the Current Period, Income Tax | 0.4 | | 0.2 | | 0.6 | 0.5 |
Amounts Reclassified to Net Income (Loss), Pretax | 10.2 | | 22.7 | | 21.5 | 37.7 |
Amounts Reclassified to Net Income (Loss), Income Tax | (2.4) | | (3.4) | | (5.1) | (6.8) |
Other comprehensive income (loss) | 5.9 | | 20.8 | | 13.4 | 31.7 |
Benefit plan adjustments(a) | Reclassification out of accumulated other comprehensive income | | | | | | |
Other Comprehensive Income Loss [Line Items] | | | | | | |
Cost of revenues | 1.9 | | 2.1 | | 3.8 | 4.5 |
Selling, general and administrative expenses | 0.6 | | 0.6 | | 1.2 | 1.2 |
Interest and other income (expense) | 7.8 | | 10 | | 17.5 | 21.7 |
Foreign currency translation adjustments | | | | | | |
Other Comprehensive Income Loss [Line Items] | | | | | | |
Amounts Arising During the Current Period, Pretax | 9.1 | | (139) | | 9.7 | (138) |
Amounts Arising During the Current Period, Income Tax | 0 | | 0 | | 0 | 0 |
Amounts Reclassified to Net Income (Loss), Pretax | 0 | | 107.2 | | 0 | 107.2 |
Amounts Reclassified to Net Income (Loss), Income Tax | 0 | | (0.5) | | 0 | (0.5) |
Other comprehensive income (loss) | 9.1 | | (32.3) | | 9.7 | (31.3) |
Gains (losses) on cash flow hedges(b) | | | | | | |
Other Comprehensive Income Loss [Line Items] | | | | | | |
Amounts Arising During the Current Period, Pretax | (14.2) | | 0.2 | | (19.5) | 0.6 |
Amounts Arising During the Current Period, Income Tax | 3.9 | | (0.1) | | 5 | (0.2) |
Amounts Reclassified to Net Income (Loss), Pretax | 3.6 | | 0 | | 1 | 0 |
Amounts Reclassified to Net Income (Loss), Income Tax | (1.2) | | 0 | | (0.3) | 0 |
Other comprehensive income (loss) | $ (7.9) | | $ 0.1 | | $ (13.8) | $ 0.4 |