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     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 9.0 $ (11.4) $ 12.2 21.7 $ 12.2 $ 34.3 9.8 68.2
Cost of revenues 652.6     666.4     2,013.0 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)     (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     14.0     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     0.0 0.0
Amounts Reclassified to Net Income (Loss), Pretax 0.0     0.0     107.2 0.0
Amounts Reclassified to Net Income (Loss), Income Tax 0.0     0.0     (0.5) 0.0
Other comprehensive income (0.6)     14.6     (32.0) 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 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     0.0     (0.2) 0.0
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     0.0 0.0
Other comprehensive income 0.0     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     0.0 (0.1)
Other operating income (expense) 0.0     0.1     0.0 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     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 9.0 $ (10.4) $ 11.1 19.6 $ 14.0 $ 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     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 0.0     0.2     0.0 0.5
Amounts Reclassified to Net Income (Loss), Income Tax 0.0     0.0     0.0 0.0
Other comprehensive income 0.0     0.2     0.0 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     0.0 0.0
Amounts Reclassified to Net Income (Loss), Pretax 0.6     0.0     0.6 0.0
Amounts Reclassified to Net Income (Loss), Income Tax 0.0     0.0     0.0 0.0
Other comprehensive income 0.0     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     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     0.0 0.0
Other comprehensive income 0.0     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)     (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     0.0 0.0
Amounts Reclassified to Net Income (Loss), Pretax 0.6     0.0     107.8 0.0
Amounts Reclassified to Net Income (Loss), Income Tax 0.0     0.0     (0.5) 0.0
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     0.0     (0.2) 0.0
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     0.0 0.0
Other comprehensive income $ 0.0     $ 0.0     $ 0.4 $ (0.1)