Fair value of financial instruments - recurring (Details) - Recurring - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Assets:
Cash and cash equivalents	$ 38.0	$ 74.9
Restricted cash	0.5	7.7
Derivative instruments asset		0.7
Total	38.5	83.3
Liabilities:
Derivative instruments liability	31.4	27.9
Total	31.4	27.9
Level 1
Assets:
Cash and cash equivalents	38.0	74.9
Restricted cash	0.5	7.7
Total	38.5	82.6
Level 2
Assets:
Derivative instruments asset		0.7
Total		0.7
Liabilities:
Derivative instruments liability	26.2	24.7
Total	26.2	24.7
Level 3
Liabilities:
Derivative instruments liability	5.2	3.2
Total	$ 5.2	$ 3.2