Fair value of financial instruments - recurring (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Assets:
Restricted cash	$ 14.1	$ 13.3
Recurring
Assets:
Cash and cash equivalents	104.4	85.6
Restricted cash	14.1	13.3
Derivative instruments asset	5.6	8.6
Total	124.1	107.5
Liabilities:
Derivative instruments liability	30.7	28.9
Total	30.7	28.9
Recurring | Level 1
Assets:
Cash and cash equivalents	104.4	85.6
Restricted cash	14.1	13.3
Total	118.5	98.9
Recurring | Level 2
Assets:
Derivative instruments asset	5.6	8.6
Total	5.6	8.6
Liabilities:
Derivative instruments liability	30.7	28.9
Total	$ 30.7	$ 28.9