Accumulated other comprehensive income (loss) - Amounts in OCI (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	$ (137.5)	$ 3.6	$ (137.1)	$ 29.3
Amounts Arising During the Current Period, Income Tax	0.1	0.0	0.3	0.1
Amounts Reclassified to Net Income (Loss), Pretax	129.9	13.0	144.9	26.0
Amounts Reclassified to Net Income (Loss), Income Tax	(3.9)	(4.4)	(7.3)	(8.9)
Other comprehensive income (loss)	(11.4)	12.2	0.8	46.5
Cost of revenues	666.8	628.9	1,360.4	1,239.2
Selling, general and administrative expenses	119.9	122.8	243.0	229.9
Interest and other income (expense)	8.1	11.4	21.2	22.6
Other operating income (expense)	1.3	5.9	(1.1)	6.0
Benefit plan adjustments
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	1.3	(2.8)	0.3	(4.3)
Amounts Arising During the Current Period, Income Tax	0.2	0.2	0.5	0.4
Amounts Reclassified to Net Income (Loss), Pretax	22.9	13.2	37.7	25.8
Amounts Reclassified to Net Income (Loss), Income Tax	(3.4)	(4.5)	(6.8)	(9.0)
Other comprehensive income (loss)	21.0	6.1	31.7	12.9
Foreign currency translation adjustments
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	(138.2)	7.6	(138.1)	33.9
Amounts Arising During the Current Period, Income Tax	0.0	0.0	0.0	0.0
Amounts Reclassified to Net Income (Loss), Pretax	107.2	0.0	107.2	0.0
Amounts Reclassified to Net Income (Loss), Income Tax	(0.5)	0.0	(0.5)	0.0
Other comprehensive income (loss)	(31.5)	7.6	(31.4)	33.9
Unrealized gains (losses) on available-for-sale securities
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax		0.7		0.9
Amounts Arising During the Current Period, Income Tax		(0.2)		(0.3)
Amounts Reclassified to Net Income (Loss), Pretax		(0.2)		(0.2)
Amounts Reclassified to Net Income (Loss), Income Tax		0.1		0.1
Other comprehensive income (loss)		0.4	0.0	0.5
Gains (losses) on cash flow hedges
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	0.2	0.0	0.6	(0.2)
Amounts Arising During the Current Period, Income Tax	(0.1)	0.0	(0.2)	0.0
Amounts Reclassified to Net Income (Loss), Pretax	0.0	(0.1)	0.0	0.1
Amounts Reclassified to Net Income (Loss), Income Tax	0.0	0.0	0.0	0.0
Other comprehensive income (loss)	0.1	(0.1)	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	0.1	0.0	0.1
Other operating income (expense)	0.0	(0.2)	0.0	0.0
AOCI Attributable to Parent
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	(136.7)	5.5	(137.2)	30.3
Amounts Arising During the Current Period, Income Tax	0.1	0.0	0.3	0.1
Amounts Reclassified to Net Income (Loss), Pretax	130.1	12.9	144.9	25.7
Amounts Reclassified to Net Income (Loss), Income Tax	(3.9)	(4.4)	(7.3)	(8.9)
Other comprehensive income (loss)	(10.4)	14.0	0.7	47.2
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	(0.2)	0.1	0.0	0.3
Amounts Reclassified to Net Income (Loss), Income Tax	0.0	0.0	0.0	0.0
Other comprehensive income (loss)	(0.2)	0.1	0.0	0.3
Foreign currency translation adjustments
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	(0.8)	(1.9)	0.1	(1.0)
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	0.0	0.0	0.0
Amounts Reclassified to Net Income (Loss), Income Tax	0.0	0.0	0.0	0.0
Other comprehensive income (loss)	(0.8)	(1.9)	0.1	(1.0)
AOCI Attributable to Noncontrolling Interest
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	(0.8)	(1.9)	0.1	(1.0)
Amounts Arising During the Current Period, Income Tax	0.0	0.0	0.0	0.0
Amounts Reclassified to Net Income (Loss), Pretax	(0.2)	0.1	0.0	0.3
Amounts Reclassified to Net Income (Loss), Income Tax	0.0	0.0	0.0	0.0
Other comprehensive income (loss)	(1.0)	(1.8)	0.1	(0.7)
Benefit plan adjustments(a)
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	1.3	(2.8)	0.3	(4.3)
Amounts Arising During the Current Period, Income Tax	0.2	0.2	0.5	0.4
Amounts Reclassified to Net Income (Loss), Pretax	22.7	13.3	37.7	26.1
Amounts Reclassified to Net Income (Loss), Income Tax	(3.4)	(4.5)	(6.8)	(9.0)
Other comprehensive income (loss)	20.8	6.2	31.7	13.2
Benefit plan adjustments(a) \| Reclassification out of accumulated other comprehensive income
Other Comprehensive Income Loss [Line Items]
Cost of revenues	2.1	2.4	4.5	4.7
Selling, general and administrative expenses	0.6	0.5	1.2	1.1
Interest and other income (expense)	(10.0)	(12.1)	(21.7)	(23.7)
Foreign currency translation adjustments
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	(139.0)	5.7	(138.0)	32.9
Amounts Arising During the Current Period, Income Tax	0.0	0.0	0.0	0.0
Amounts Reclassified to Net Income (Loss), Pretax	107.2	0.0	107.2	0.0
Amounts Reclassified to Net Income (Loss), Income Tax	(0.5)	0.0	(0.5)	0.0
Other comprehensive income (loss)	(32.3)	5.7	(31.3)	32.9
Unrealized gains (losses) on available-for-sale securities(b)
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax		0.7		0.9
Amounts Arising During the Current Period, Income Tax		(0.2)		(0.3)
Amounts Reclassified to Net Income (Loss), Pretax		(0.2)		(0.2)
Amounts Reclassified to Net Income (Loss), Income Tax		0.1		0.1
Other comprehensive income (loss)		0.4		0.5
Gains (losses) on cash flow hedges(c)
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	0.2	0.0	0.6	(0.2)
Amounts Arising During the Current Period, Income Tax	(0.1)	0.0	(0.2)	0.0
Amounts Reclassified to Net Income (Loss), Pretax	0.0	(0.1)	0.0	0.1
Amounts Reclassified to Net Income (Loss), Income Tax	0.0	0.0	0.0	0.0
Other comprehensive income (loss)	$ 0.1	$ (0.1)	$ 0.4	$ (0.1)

The aggregate cost of goods produced and sold and services rendered during the reporting period.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.2)
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

+ Details

Name:	us-gaap_CostOfRevenue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The aggregate amount of income or expense from ancillary business-related activities (that is to say, excluding major activities considered part of the normal operations of the business).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.7)
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

+ Details

Name:	us-gaap_NonoperatingIncomeExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount before tax and reclassification adjustments of other comprehensive income (loss).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 14A
-URI http://asc.fasb.org/extlink&oid=84228862&loc=SL7669686-108580

+ Details

Name:	us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsBeforeTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of tax expense (benefit) allocated to other comprehensive income (loss) before reclassification adjustment from accumulated other comprehensive income (loss).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 20
-Section 45
-Paragraph 11
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=79507043&loc=d3e39076-109324

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 12
-URI http://asc.fasb.org/extlink&oid=84228862&loc=d3e640-108580

+ Details

Name:	us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount after tax and reclassification adjustments of other comprehensive income (loss).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21463-112644

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=84228862&loc=SL7669619-108580

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=84228862&loc=SL7669625-108580

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 55
-Paragraph 15
-URI http://asc.fasb.org/extlink&oid=84158357&loc=d3e1436-108581

+ Details

Name:	us-gaap_OtherComprehensiveIncomeLossNetOfTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The net amount of other operating income and expenses, the components of which are not separately disclosed on the income statement, from items that are associated with the entity's normal revenue producing operations.

+ References

No definition available.

+ Details

Name:	us-gaap_OtherOperatingIncomeExpenseNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of selling, general and administrative expense classified as other.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.4)
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

+ Details

Name:	us-gaap_OtherSellingGeneralAndAdministrativeExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount before tax of reclassification adjustments of other comprehensive income (loss).

+ References

+ Details

Name:	us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodBeforeTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of tax expense (benefit) of reclassification adjustment from accumulated other comprehensive income (loss).

+ References

+ Details

Name:	us-gaap_ReclassificationFromAociCurrentPeriodTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_StatementEquityComponentsAxis=us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetUnamortizedGainLossMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementEquityComponentsAxis=us-gaap_AccumulatedTranslationAdjustmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementEquityComponentsAxis=us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementEquityComponentsAxis=us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis=us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementEquityComponentsAxis=us-gaap_AccumulatedOtherComprehensiveIncomeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementEquityComponentsAxis=us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetGainLossAttributableToNoncontrollingInterestMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementEquityComponentsAxis=us-gaap_AccumulatedForeignCurrencyAdjustmentAttributableToNoncontrollingInterestMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementEquityComponentsAxis=us-gaap_AociAttributableToNoncontrollingInterestMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementEquityComponentsAxis=us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetGainLossIncludingPortionAttributableToNoncontrollingInterestMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementEquityComponentsAxis=us-gaap_AccumulatedForeignCurrencyAdjustmentIncludingPortionAttributableToNoncontrollingInterestMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementEquityComponentsAxis=us-gaap_AccumulatedNetInvestmentGainLossIncludingPortionAttributableToNoncontrollingInterestMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementEquityComponentsAxis=us-gaap_AccumulatedNetGainLossFromCashFlowHedgesIncludingPortionAttributableToNoncontrollingInterestMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: