Fair value of financial instruments - recurring (Details) - Recurring - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Assets:
Cash and cash equivalents	$ 38.8	$ 74.9
Restricted cash	7.1	7.7
Derivative instruments asset	0.4	0.7
Total	46.3	83.3
Liabilities:
Derivative instruments liability	19.1	27.9
Total	19.1	27.9
Level 1
Assets:
Cash and cash equivalents	38.8	74.9
Restricted cash	7.1	7.7
Total	45.9	82.6
Level 2
Assets:
Derivative instruments asset	0.4	0.7
Total	0.4	0.7
Liabilities:
Derivative instruments liability	17.6	24.7
Total	17.6	24.7
Level 3
Liabilities:
Derivative instruments liability	1.5	3.2
Total	$ 1.5	$ 3.2