Accounting for derivative instruments and hedging activities - AOCI (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
AOCI Attributable to Parent, Net of Tax
Change in fair value of cash flow hedges	$ (0.5)	$ (0.3)
Realized from OCI during the period	(0.5)	(0.3)
Interest rate swaps
AOCI Attributable to Parent, Net of Tax
Accumulated OCI balance at beginning of period	1.6	1.6
Change in fair value of cash flow hedges	(0.5)	(0.3)
Realized from OCI during the period	0.5	0.3
Accumulated OCI balance at end of period	1.6	$ 1.6
Settlements expected to be recognized from OCI in expense in the next 12 months, net of tax	0.5
Tax effect of gains (losses) expected to be realized from OCI in the next 12 months	$ 0.1