Stockholders' Deficit - Components of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance	$ (149.7)	$ (39.5)	$ (374.2)	$ (309.8)	$ (39.5)	$ (309.8)
Amounts reclassified from AOCL into earnings, net of tax	(2.3)		(0.6)		(6.7)	0.5
Ending balance	(73.4)	(149.7)	30.2	(374.2)	(73.4)	30.2
Investment Securities
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance	30.4	24.7	11.9	(1.1)	24.7	(1.1)
Unrealized gains/(losses)	24.6	7.2	13.0	16.8
Tax benefit/(expense)	(4.8)	(1.1)	(2.7)	(3.8)
Amounts reclassified from AOCL into earnings, net of tax	(0.3)	(0.4)	(0.4)		(0.7)	(0.4)
Ending balance	49.9	30.4	21.8	11.9	49.9	21.8
Hedging Activities
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance	17.1	(3.6)	11.3	7.4	(3.6)	7.4
Unrealized gains/(losses)	(15.1)	27.3	(3.9)	4.4
Tax benefit/(expense)	0.1	(0.3)	0.1	0.9
Amounts reclassified from AOCL into earnings, net of tax	(4.1)	(6.3)	(2.1)	(1.4)	(10.4)	(3.5)
Ending balance	(2.0)	17.1	5.4	11.3	(2.0)	5.4
Foreign Currency Translation
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance	(101.2)	(101.2)	(101.2)	(101.2)	(101.2)	(101.2)
Ending balance	(101.2)	(101.2)	(101.2)	(101.2)	(101.2)	(101.2)
Defined benefit plans
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance	(126.6)	(128.9)	(133.6)	(136.1)	(128.9)	(136.1)
Amounts reclassified from AOCL into earnings, net of tax	2.1	2.3	1.9	2.5	4.4	4.4
Ending balance	(124.5)	(126.6)	(131.7)	(133.6)	(124.5)	(131.7)
Total AOCL
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance	(180.3)	(209.0)	(211.6)	(231.0)	(209.0)	(231.0)
Unrealized gains/(losses)	9.5	34.5	9.1	21.2
Tax benefit/(expense)	(4.7)	(1.4)	(2.6)	(2.9)
Amounts reclassified from AOCL into earnings, net of tax	(2.3)	(4.4)	(0.6)	1.1
Ending balance	$ (177.8)	$ (180.3)	$ (205.7)	$ (211.6)	$ (177.8)	$ (205.7)

No definition available.

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

- Definition

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 14A
-URI http://asc.fasb.org/extlink&oid=118930883&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).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 20
-Section 45
-Paragraph 11
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109238882&loc=d3e39076-109324

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 20
-Section 45
-Paragraph 2
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=109238882&loc=d3e38679-109324

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 12
-URI http://asc.fasb.org/extlink&oid=118930883&loc=d3e640-108580

+ Details

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

- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 17B
-URI http://asc.fasb.org/extlink&oid=118930883&loc=SL34724394-108580

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 14A
-URI http://asc.fasb.org/extlink&oid=118930883&loc=SL7669686-108580

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 55
-Paragraph 15
-URI http://asc.fasb.org/extlink&oid=118951672&loc=d3e1436-108581

+ Details

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

- Definition

Amount of stockholders' equity (deficit), net of receivables from officers, directors, owners, and affiliates of the entity, attributable to both the parent and noncontrolling interests. Amount excludes temporary equity. Alternate caption for the concept is permanent equity.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 45
-Paragraph 16
-URI http://asc.fasb.org/extlink&oid=108774443&loc=SL4568740-111683

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 55
-Paragraph 4I
-URI http://asc.fasb.org/extlink&oid=116874947&loc=SL4590271-111686

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 45
-Paragraph 15
-URI http://asc.fasb.org/extlink&oid=108774443&loc=SL4568447-111683

+ Details

Name:	us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

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

- Details

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

- Details

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