Segment and geographic information - Geographic (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Revenue and Assets
Revenues	$ 108.6	$ 101.2	$ 331.0	$ 305.8
Property, plant, and equipment, net (Note 4)	652.6		652.6		$ 733.2
United States
Revenue and Assets
Revenues	75.1	65.6	203.2	183.8
Property, plant, and equipment, net (Note 4)	437.0		437.0		499.2
Canada
Revenue and Assets
Revenues	33.5	$ 35.6	127.8	$ 122.0
Property, plant, and equipment, net (Note 4)	$ 215.6		$ 215.6		$ 234.0