Accounting for derivative instruments - gain, loss (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Impact of derivative instruments on the consolidated statements of operations
Change in fair value of derivative instruments	$ (1.9)	$ 9.0	$ (5.8)	$ 20.0
Derivative instruments not designated as cash flow hedges
Impact of derivative instruments on the consolidated statements of operations
Change in fair value of derivative instruments	(1.9)	9.0	(5.8)	20.0
Derivative instruments not designated as cash flow hedges | Natural gas swaps
Impact of derivative instruments on the consolidated statements of operations
Realized (gains) and losses of derivative instruments	(0.7)	0.6	(1.2)	4.0
Change in fair value of derivative instruments	0.2	0.2	(0.3)	6.0
Derivative instruments not designated as cash flow hedges | Gas purchase agreements
Impact of derivative instruments on the consolidated statements of operations
Realized (gains) and losses of derivative instruments	3.1	12.4	7.9	36.4
Change in fair value of derivative instruments	(1.3)	5.6	(4.3)	16.8
Derivative instruments not designated as cash flow hedges | Interest rate swaps
Impact of derivative instruments on the consolidated statements of operations
Realized (gains) and losses of derivative instruments	0.9	1.0	2.6	2.7
Change in fair value of derivative instruments	(0.8)	$ 3.2	(1.2)	$ (2.8)
Derivative instruments not designated as cash flow hedges | Foreign currency forward contracts
Impact of derivative instruments on the consolidated statements of operations
Change in fair value of derivative instruments	$ (0.2)		$ (0.7)