Fair value of financial instruments - recurring (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Assets:
Restricted cash	$ 1.9	$ 6.2
Recurring
Assets:
Cash and cash equivalents	80.8	78.7
Restricted cash	1.9	6.2
Derivative instruments asset	8.5	5.5
Total	91.2	90.4
Liabilities:
Derivative instruments liability	23.8	24.3
Total	23.8	24.3
Recurring | Level 1
Assets:
Cash and cash equivalents	80.8	78.7
Restricted cash	1.9	6.2
Total	82.7	84.9
Recurring | Level 2
Assets:
Derivative instruments asset	8.5	5.5
Total	8.5	5.5
Liabilities:
Derivative instruments liability	21.5	24.3
Total	21.5	$ 24.3
Recurring | Level 3
Liabilities:
Derivative instruments liability	2.3
Total	$ 2.3