Summary of significant accounting policies - Deferred software costs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Summary of significant accounting policies
Unamortized deferred software costs	$ 173.5	$ 189.3
Amortization of intangible assets	$ 15.8	10.2
Minimum
Summary of significant accounting policies
Amortization straight-line basis and over useful life	3 years 3 months 18 days
Maximum
Summary of significant accounting policies
Amortization straight-line basis and over useful life	20 years 10 months 24 days
Deferred software costs
Summary of significant accounting policies
Unamortized deferred software costs	$ 9.3	10.5
Amortization of intangible assets	$ 4.8	$ 4.6	$ 3.2
Deferred software costs | Minimum
Summary of significant accounting policies
Amortization straight-line basis and over useful life	3 years
Deferred software costs | Maximum
Summary of significant accounting policies
Amortization straight-line basis and over useful life	5 years