Face (par) amount of debt instrument at time of issuance.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28551-108399

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=114775985&loc=d3e28878-108400

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

Debt - Narrative (Details) - Dec. 31, 2018 - Senior notes	USD ($)	GBP (£)	EUR (€)
Senior notes due 2025
Debt Instrument [Line Items]
Principal amount \| £		£ 250,000,000
Senior notes due 2026
Debt Instrument [Line Items]
Principal amount \| €			€ 650,000,000
Senior notes due 2021
Debt Instrument [Line Items]
Principal amount \| $	$ 500,000,000