Accumulated other comprehensive income (loss) - Amounts in OCI (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended |
Sep. 30, 2018 | Jun. 30, 2018 | Mar. 31, 2018 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Sep. 30, 2018 | Sep. 30, 2017 |
Other Comprehensive Income Loss [Line Items] | | | | | | | | |
Amounts Arising During the Current Period, Pretax | $ (2.2) | | | $ 11.8 | | | $ (139.3) | $ 41.1 |
Amounts Arising During the Current Period, Income Tax | 0.2 | | | 1 | | | 0.5 | 1.1 |
Amounts Reclassified to Net Income (Loss), Pretax | 14.3 | | | 13.6 | | | 159.2 | 39.6 |
Amounts Reclassified to Net Income (Loss), Income Tax | (3.3) | | | (4.7) | | | (10.6) | (13.6) |
Other comprehensive income | 9 | $ (11.4) | $ 12.2 | 21.7 | $ 12.2 | $ 34.3 | 9.8 | 68.2 |
Cost of revenues | 652.6 | | | 666.4 | | | 2,013 | 1,905.6 |
Selling, general and administrative expenses | 125.4 | | | 116.6 | | | 368.4 | 346.5 |
Interest and other income (expense) | (8.1) | | | (21.2) | | | (29.3) | (43.8) |
Other operating income (expense) | 7.4 | | | 0.1 | | | 6.3 | 6.1 |
Benefit plan adjustments | | | | | | | | |
Other Comprehensive Income Loss [Line Items] | | | | | | | | |
Amounts Arising During the Current Period, Pretax | (1.1) | | | (5) | | | (0.8) | (9.3) |
Amounts Arising During the Current Period, Income Tax | 0.2 | | | 1.1 | | | 0.7 | 1.5 |
Amounts Reclassified to Net Income (Loss), Pretax | 13.8 | | | 14 | | | 51.5 | 39.8 |
Amounts Reclassified to Net Income (Loss), Income Tax | (3.3) | | | (4.9) | | | (10.1) | (13.9) |
Other comprehensive income | 9.6 | | | 5.2 | | | 41.3 | 18.1 |
Foreign currency translation adjustments | | | | | | | | |
Other Comprehensive Income Loss [Line Items] | | | | | | | | |
Amounts Arising During the Current Period, Pretax | (0.6) | | | 14.6 | | | (138.7) | 48.5 |
Amounts Arising During the Current Period, Income Tax | 0 | | | 0 | | | 0 | 0 |
Amounts Reclassified to Net Income (Loss), Pretax | 0 | | | 0 | | | 107.2 | 0 |
Amounts Reclassified to Net Income (Loss), Income Tax | 0 | | | 0 | | | (0.5) | 0 |
Other comprehensive income | (0.6) | | | 14.6 | | | (32) | 48.5 |
Unrealized gains (losses) on available-for-sale securities | | | | | | | | |
Other Comprehensive Income Loss [Line Items] | | | | | | | | |
Amounts Arising During the Current Period, Pretax | | | | 0.4 | | | | 1.3 |
Amounts Arising During the Current Period, Income Tax | | | | (0.1) | | | | (0.4) |
Amounts Reclassified to Net Income (Loss), Pretax | | | | (0.7) | | | | (0.9) |
Amounts Reclassified to Net Income (Loss), Income Tax | | | | 0.2 | | | | 0.3 |
Other comprehensive income | | | | (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 | | | (0.1) | | | 0.7 | (0.3) |
Amounts Arising During the Current Period, Income Tax | 0 | | | 0 | | | (0.2) | 0 |
Amounts Reclassified to Net Income (Loss), Pretax | (0.1) | | | 0.1 | | | (0.1) | 0.2 |
Amounts Reclassified to Net Income (Loss), Income Tax | 0 | | | 0 | | | 0 | 0 |
Other comprehensive income | 0 | | | 0 | | | 0.4 | (0.1) |
Gains (losses) on cash flow hedges | Reclassification out of accumulated other comprehensive income | | | | | | | | |
Other Comprehensive Income Loss [Line Items] | | | | | | | | |
Interest and other income (expense) | 0 | | | 0 | | | 0 | (0.1) |
Other operating income (expense) | 0 | | | 0.1 | | | 0 | 0.1 |
AOCI Attributable to Parent | | | | | | | | |
Other Comprehensive Income Loss [Line Items] | | | | | | | | |
Amounts Arising During the Current Period, Pretax | (1.6) | | | 9.9 | | | (138.8) | 40.2 |
Amounts Arising During the Current Period, Income Tax | 0.2 | | | 1 | | | 0.5 | 1.1 |
Amounts Reclassified to Net Income (Loss), Pretax | 13.7 | | | 13.4 | | | 158.6 | 39.1 |
Amounts Reclassified to Net Income (Loss), Income Tax | (3.3) | | | (4.7) | | | (10.6) | (13.6) |
Other comprehensive income | 9 | $ (10.4) | $ 11.1 | 19.6 | $ 14 | $ 33.2 | 9.7 | 66.8 |
Benefit plan adjustments | | | | | | | | |
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 | | | 0.2 | | | 0 | 0.5 |
Amounts Reclassified to Net Income (Loss), Income Tax | 0 | | | 0 | | | 0 | 0 |
Other comprehensive income | 0 | | | 0.2 | | | 0 | 0.5 |
Foreign currency translation adjustments | | | | | | | | |
Other Comprehensive Income Loss [Line Items] | | | | | | | | |
Amounts Arising During the Current Period, Pretax | (0.6) | | | 1.9 | | | (0.5) | 0.9 |
Amounts Arising During the Current Period, Income Tax | 0 | | | 0 | | | 0 | 0 |
Amounts Reclassified to Net Income (Loss), Pretax | 0.6 | | | 0 | | | 0.6 | 0 |
Amounts Reclassified to Net Income (Loss), Income Tax | 0 | | | 0 | | | 0 | 0 |
Other comprehensive income | 0 | | | 1.9 | | | 0.1 | 0.9 |
AOCI Attributable to Noncontrolling Interest | | | | | | | | |
Other Comprehensive Income Loss [Line Items] | | | | | | | | |
Amounts Arising During the Current Period, Pretax | (0.6) | | | 1.9 | | | (0.5) | 0.9 |
Amounts Arising During the Current Period, Income Tax | 0 | | | 0 | | | 0 | 0 |
Amounts Reclassified to Net Income (Loss), Pretax | 0.6 | | | 0.2 | | | 0.6 | 0.5 |
Amounts Reclassified to Net Income (Loss), Income Tax | 0 | | | 0 | | | 0 | 0 |
Other comprehensive income | 0 | | | 2.1 | | | 0.1 | 1.4 |
Benefit plan adjustments(a) | | | | | | | | |
Other Comprehensive Income Loss [Line Items] | | | | | | | | |
Amounts Arising During the Current Period, Pretax | (1.1) | | | (5) | | | (0.8) | (9.3) |
Amounts Arising During the Current Period, Income Tax | 0.2 | | | 1.1 | | | 0.7 | 1.5 |
Amounts Reclassified to Net Income (Loss), Pretax | 13.8 | | | 14.2 | | | 51.5 | 40.3 |
Amounts Reclassified to Net Income (Loss), Income Tax | (3.3) | | | (4.9) | | | (10.1) | (13.9) |
Other comprehensive income | 9.6 | | | 5.4 | | | 41.3 | 18.6 |
Benefit plan adjustments(a) | Reclassification out of accumulated other comprehensive income | | | | | | | | |
Other Comprehensive Income Loss [Line Items] | | | | | | | | |
Cost of revenues | 1.9 | | | 2.2 | | | 6.4 | 6.9 |
Selling, general and administrative expenses | 0.7 | | | 0.6 | | | 1.9 | 1.7 |
Interest and other income (expense) | 8.9 | | | 11.7 | | | 30.6 | 35.4 |
Foreign currency translation adjustments | | | | | | | | |
Other Comprehensive Income Loss [Line Items] | | | | | | | | |
Amounts Arising During the Current Period, Pretax | (1.2) | | | 16.5 | | | (139.2) | 49.4 |
Amounts Arising During the Current Period, Income Tax | 0 | | | 0 | | | 0 | 0 |
Amounts Reclassified to Net Income (Loss), Pretax | 0.6 | | | 0 | | | 107.8 | 0 |
Amounts Reclassified to Net Income (Loss), Income Tax | 0 | | | 0 | | | (0.5) | 0 |
Other comprehensive income | (0.6) | | | 16.5 | | | (31.9) | 49.4 |
Unrealized gains (losses) on available-for-sale securities(b) | | | | | | | | |
Other Comprehensive Income Loss [Line Items] | | | | | | | | |
Amounts Arising During the Current Period, Pretax | | | | 0.4 | | | | 1.3 |
Amounts Arising During the Current Period, Income Tax | | | | (0.1) | | | | (0.4) |
Amounts Reclassified to Net Income (Loss), Pretax | | | | (0.7) | | | | (0.9) |
Amounts Reclassified to Net Income (Loss), Income Tax | | | | 0.2 | | | | 0.3 |
Other comprehensive income | | | | (0.2) | | | | 0.3 |
Gains (losses) on cash flow hedges(c) | | | | | | | | |
Other Comprehensive Income Loss [Line Items] | | | | | | | | |
Amounts Arising During the Current Period, Pretax | 0.1 | | | (0.1) | | | 0.7 | (0.3) |
Amounts Arising During the Current Period, Income Tax | 0 | | | 0 | | | (0.2) | 0 |
Amounts Reclassified to Net Income (Loss), Pretax | (0.1) | | | 0.1 | | | (0.1) | 0.2 |
Amounts Reclassified to Net Income (Loss), Income Tax | 0 | | | 0 | | | 0 | 0 |
Other comprehensive income | $ 0 | | | $ 0 | | | $ 0.4 | $ (0.1) |