Fair value measurements - Fair Value Measurements on a Non-Recurring Basis (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Fair Value Measurements on a Non-Recurring Basis
Impairment of indefinite-lived intangible asset excluding goodwill	$ (8.0)	$ (5.0)
Trade Names
Fair Value Measurements on a Non-Recurring Basis
Impairment of indefinite-lived intangible asset excluding goodwill	(13.0)	(5.0)
Trade Names \| Impairment of Intangible Assets
Fair Value Measurements on a Non-Recurring Basis
Impairment of indefinite-lived intangible asset excluding goodwill	$ 8.0	$ 5.0

Amount of impairment loss resulting from write-down of assets, excluding financial assets and goodwill, lacking physical substance and having a projected indefinite period of benefit to fair value.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16373-109275

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585

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