Accounting for derivative instruments - FV by Bal Sht Loc (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Fair value of derivative instruments
Derivative Assets	$ 5.6	$ 8.9
Derivative Liabilities	30.7	29.2
Derivative instruments designated as cash flow hedges
Fair value of derivative instruments
Derivative Liabilities	2.5	2.8
Derivative instruments designated as cash flow hedges | Interest rate swaps | Current
Fair value of derivative instruments
Derivative Liabilities	0.7	0.8
Derivative instruments designated as cash flow hedges | Interest rate swaps | Long-term
Fair value of derivative instruments
Derivative Liabilities	1.8	2.0
Derivative instruments not designated as cash flow hedges
Fair value of derivative instruments
Derivative Assets	5.6	8.9
Derivative Liabilities	28.2	26.4
Derivative instruments not designated as cash flow hedges | Interest rate swaps | Current
Fair value of derivative instruments
Derivative Assets	1.2	0.4
Derivative Liabilities	1.5	1.9
Derivative instruments not designated as cash flow hedges | Interest rate swaps | Long-term
Fair value of derivative instruments
Derivative Assets	2.8	4.5
Derivative Liabilities	6.4	6.5
Derivative instruments not designated as cash flow hedges | Natural gas swaps | Current
Fair value of derivative instruments
Derivative Assets	1.6	3.9
Derivative Liabilities	1.0	0.8
Derivative instruments not designated as cash flow hedges | Natural gas swaps | Long-term
Fair value of derivative instruments
Derivative Assets		0.1
Derivative Liabilities	0.3
Derivative instruments not designated as cash flow hedges | Gas purchase agreements | Current
Fair value of derivative instruments
Derivative Liabilities	2.6	4.5
Derivative instruments not designated as cash flow hedges | Gas purchase agreements | Long-term
Fair value of derivative instruments
Derivative Liabilities	15.9	$ 12.7
Derivative instruments not designated as cash flow hedges | Foreign currency forward contracts | Current
Fair value of derivative instruments
Derivative Liabilities	$ 0.5