Significant accounting policies - Investments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Sep. 30, 2018	Sep. 30, 2016
Investments
Impairment on investments	$ 0.0	$ 0.0	$ 0.0
Minority interest in preferred stock in privately held company
Investments
Purchase price of investment					$ 0.3
Investment in a convertible notes of a privately held company
Investments
Investment principle amount				$ 0.2