Accumulated other comprehensive (loss) - Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	$ 3.6	$ (5.2)	$ 29.3	$ 10.7
Amounts Arising During the Current Period, Income Tax	0.0	0.4	0.1	0.8
Amounts Reclassified to Net Income (Loss), Pretax	13.0	13.4	26.0	26.5
Amounts Reclassified to Net Income (Loss), Income Tax	(4.4)	(4.6)	(8.9)	(8.8)
Other comprehensive income	12.2	4.0	46.5	29.2
Cost of revenues	628.9	596.1	1,239.2	1,185.0
Selling, general and administrative expenses	122.8	105.0	229.9	213.7
Interest and other income (expense)	(11.4)	(9.4)	(22.6)	(19.1)
Other operating income (expense)	5.9	6.2	6.0	6.9
Benefit plan adjustments, Attributable to parent
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	(2.8)	(0.8)	(4.3)	(1.9)
Amounts Arising During the Current Period, Income Tax	0.2	0.2	0.4	0.5
Amounts Reclassified to Net Income (Loss), Pretax	13.2	12.6	25.8	24.9
Amounts Reclassified to Net Income (Loss), Income Tax	(4.5)	(4.5)	(9.0)	(8.7)
Other comprehensive income	6.1	7.5	12.9	14.8
Foreign currency translation adjustments, Attributable to parent
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	7.6	(3.5)	33.9	13.6
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	0.0	0.0
Amounts Reclassified to Net Income (Loss), Income Tax	0.0	0.0	0.0	0.0
Other comprehensive income	7.6	(3.5)	33.9	13.6
Unrealized gains (losses) on available-for-sale securities, Attributable to parent
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	0.7	0.3	0.9	0.5
Amounts Arising During the Current Period, Income Tax	(0.2)	(0.2)	(0.3)	(0.2)
Amounts Reclassified to Net Income (Loss), Pretax	(0.2)	(0.5)	(0.2)	(0.5)
Amounts Reclassified to Net Income (Loss), Income Tax	0.1	0.2	0.1	0.2
Other comprehensive income	0.4	(0.2)	0.5	0.0
Gains (losses) on cash flow hedges, Attributable to parent
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	0.0	(1.2)	(0.2)	(2.2)
Amounts Arising During the Current Period, Income Tax	0.0	0.4	0.0	0.5
Amounts Reclassified to Net Income (Loss), Pretax	(0.1)	1.1	0.1	1.8
Amounts Reclassified to Net Income (Loss), Income Tax	0.0	(0.3)	0.0	(0.3)
Other comprehensive income	(0.1)	0.0	(0.1)	(0.2)
Gains (losses) on cash flow hedges, Attributable to parent | Reclassification out of accumulated other comprehensive income
Other Comprehensive Income Loss [Line Items]
Interest and other income (expense)	(0.1)	(0.1)	(0.1)	(0.2)
Other operating income (expense)	(0.2)	0.8	0.0	1.4
AOCI Attributable to Parent
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	5.5	(5.2)	30.3	10.0
Amounts Arising During the Current Period, Income Tax	0.0	0.4	0.1	0.8
Amounts Reclassified to Net Income (Loss), Pretax	12.9	13.2	25.7	26.2
Amounts Reclassified to Net Income (Loss), Income Tax	(4.4)	(4.6)	(8.9)	(8.8)
Other comprehensive income	14.0	3.8	47.2	28.2
Benefit plan adjustments, Attributable to noncontrolling interest
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	0.1	0.2	0.3	0.3
Amounts Reclassified to Net Income (Loss), Income Tax	0.0	0.0	0.0	0.0
Other comprehensive income	0.1	0.2	0.3	0.3
Foreign currency translation adjustments, Attributable to noncontrolling interest
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	(1.9)	0.0	(1.0)	0.7
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	0.0	0.0
Amounts Reclassified to Net Income (Loss), Income Tax	0.0	0.0	0.0	0.0
Other comprehensive income	(1.9)	0.0	(1.0)	0.7
AOCI Attributable to Noncontrolling Interest
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	(1.9)	0.0	(1.0)	0.7
Amounts Arising During the Current Period, Income Tax	0.0	0.0	0.0	0.0
Amounts Reclassified to Net Income (Loss), Pretax	0.1	0.2	0.3	0.3
Amounts Reclassified to Net Income (Loss), Income Tax	0.0	0.0	0.0	0.0
Other comprehensive income	(1.8)	0.2	(0.7)	1.0
Benefit plan adjustments
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	(2.8)	(0.8)	(4.3)	(1.9)
Amounts Arising During the Current Period, Income Tax	0.2	0.2	0.4	0.5
Amounts Reclassified to Net Income (Loss), Pretax	13.3	12.8	26.1	25.2
Amounts Reclassified to Net Income (Loss), Income Tax	(4.5)	(4.5)	(9.0)	(8.7)
Other comprehensive income	6.2	7.7	13.2	15.1
Benefit plan adjustments | Reclassification out of accumulated other comprehensive income
Other Comprehensive Income Loss [Line Items]
Cost of revenues	2.4	2.1	4.7	4.4
Selling, general and administrative expenses	0.5	0.6	1.1	1.1
Interest and other income (expense)	12.1	10.1	23.7	19.8
Foreign currency translation adjustments
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	5.7	(3.5)	32.9	14.3
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	0.0	0.0
Amounts Reclassified to Net Income (Loss), Income Tax	0.0	0.0	0.0	0.0
Other comprehensive income	5.7	(3.5)	32.9	14.3
Unrealized gains (losses) on available-for-sale securities(b)
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	0.7	0.3	0.9	0.5
Amounts Arising During the Current Period, Income Tax	(0.2)	(0.2)	(0.3)	(0.2)
Amounts Reclassified to Net Income (Loss), Pretax	(0.2)	(0.5)	(0.2)	(0.5)
Amounts Reclassified to Net Income (Loss), Income Tax	0.1	0.2	0.1	0.2
Other comprehensive income	0.4	(0.2)	0.5	0.0
Gains (losses) on cash flow hedges(c)
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	0.0	(1.2)	(0.2)	(2.2)
Amounts Arising During the Current Period, Income Tax	0.0	0.4	0.0	0.5
Amounts Reclassified to Net Income (Loss), Pretax	(0.1)	1.1	0.1	1.8
Amounts Reclassified to Net Income (Loss), Income Tax	0.0	(0.3)	0.0	(0.3)
Other comprehensive income	$ (0.1)	$ 0.0	$ (0.1)	$ (0.2)