Segment and geographic information - Geographic (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Revenue and Assets
Revenues	$ 124.0	$ 98.2	$ 222.4	$ 204.6
Property, plant, and equipment, net	705.8		705.8		$ 733.2
United States
Revenue and Assets
Revenues	68.6	59.4	128.3	118.1
Property, plant, and equipment, net	486.7		486.7		499.2
Canada
Revenue and Assets
Revenues	55.4	$ 38.8	94.1	$ 86.5
Property, plant, and equipment, net	$ 219.1		$ 219.1		$ 234.0