Accounting for derivative instruments - AOCI (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
AOCI Attributable to Parent, Net of Tax
Change in fair value of cash flow hedges			$ (0.3)	$ (0.6)
Realized from OCI during the period	$ 0.1	$ 0.2	0.5	0.5
Interest rate swaps
AOCI Attributable to Parent, Net of Tax
Accumulated OCI balance at beginning of period	0.8	(0.1)	0.7	0.2
Change in fair value of cash flow hedges			(0.3)	(0.6)
Realized from OCI during the period	0.1	0.2	0.5	0.5
Accumulated OCI balance at end of period	$ 0.9	$ 0.1	$ 0.9	$ 0.1