Fair value of financial instruments - recurring (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2016	Dec. 31, 2015
Assets:
Restricted cash	$ 14.3	$ 15.2
Recurring
Assets:
Cash and cash equivalents	154.2	72.4
Restricted cash	14.3	15.2
Derivative instruments asset	2.7	0.3
Total	171.2	87.9
Liabilities:
Derivative instruments liability	52.1	57.5
Total	52.1	57.5
Recurring | Credit valuation adjustments
Liabilities:
Net increase in fair value	5.3	3.8
Pre-tax gain in other comprehensive income	0.6	0.4
Gain in change in fair value of derivatives	4.7	3.4
Recurring | Level 1
Assets:
Cash and cash equivalents	154.2	72.4
Restricted cash	14.3	15.2
Total	168.5	87.6
Recurring | Level 2
Assets:
Derivative instruments asset	2.7	0.3
Total	2.7	0.3
Liabilities:
Derivative instruments liability	52.1	57.5
Total	$ 52.1	$ 57.5