Accounting for derivative instruments - AOCI (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
AOCI Attributable to Parent, Net of Tax
Change in fair value of cash flow hedges		$ (0.1)	$ 0.2	$ (0.3)
Realized from OCI during the period	$ 0.1	0.1	0.2	0.4
Interest rate swaps
AOCI Attributable to Parent, Net of Tax
Accumulated OCI balance at beginning of period	1.4	0.8	1.1	0.7
Change in fair value of cash flow hedges		(0.1)	0.2	(0.3)
Realized from OCI during the period	0.1	0.1	0.2	0.4
Accumulated OCI balance at end of period	$ 1.5	$ 0.8	$ 1.5	$ 0.8