Fair value of financial instruments - recurring (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Assets:
Restricted cash	$ 12.5	$ 13.3
Recurring
Assets:
Cash and cash equivalents	122.4	85.6
Restricted cash	12.5	13.3
Derivative instruments asset	5.1	8.6
Total	140.0	107.5
Liabilities:
Derivative instruments liability	33.0	28.9
Total	33.0	28.9
Recurring | Level 1
Assets:
Cash and cash equivalents	122.4	85.6
Restricted cash	12.5	13.3
Total	134.9	98.9
Recurring | Level 2
Assets:
Derivative instruments asset	5.1	8.6
Total	5.1	8.6
Liabilities:
Derivative instruments liability	33.0	28.9
Total	$ 33.0	$ 28.9