Goodwill and intangible assets - Change in goodwill, intangible assets with an indefinite life and with a finite life (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Goodwill
Balance at the beginning of the period	$ 401.0
Additions		$ 400.7
Foreign currency translation	(0.4)	0.3
Balance at the end of the period	400.6	401.0
Indefinite lived intangible Asset
Balance at the beginning of the period	27.0	5.0
Additions	3.1	27.0
Impairment of indefinite-lived intangible asset excluding goodwill	(8.0)	(5.0)
Balance at the end of the period	22.1	27.0
Finite lived intangible assets
Balance at the beginning of the period	189.3
Additions		199.5
Amortization of intangible assets	(15.8)
Balance at the end of the period	173.5	189.3
Balance at the beginning of the period	617.3	5.0
Additions	3.1	627.2
Foreign currency translation	(0.4)	0.3
Impairments	(13.0)	(5.0)
Amortization of intangible assets	(15.8)
Balance at the end of the period	$ 596.2	$ 617.3