v2.4.0.8

Derivative instruments and hedging activities (Details 5) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Impact of derivative instruments on the consolidated statements of operations
Change in fair value of derivative instruments	$ (3.5)	$ 10.7	$ 33.4	$ (51.3)
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	3.5	10.7	(33.4)	(51.3)
Derivative instruments not designated as cash flow hedges | Natural gas swaps
Impact of derivative instruments on the consolidated statements of operations
Change in fair value of derivative instruments	0.6	(1.0)	1.4	(0.2)
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	7.6	13.3	38.0	29.3
Change in fair value of derivative instruments	3.6	(10.0)	(12.0)	(49.3)
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	2.7	1.2	9.4	3.4
Change in fair value of derivative instruments	(0.7)	0.3	(22.8)	(1.8)
Derivative instruments not designated as cash flow hedges | Foreign currency forward contracts
Impact of derivative instruments on the consolidated statements of operations
Realized (gains) and losses of derivative instruments	(1.1)	(2.1)	(14.4)	(17.3)
Change in fair value of derivative instruments	$ (0.2)	$ (4.7)	$ 18.5	$ 8.2

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- Definition

The difference between the book value and the sale price of options, swaps, futures, forward contracts, and other derivative instruments. This element refers to the gain (loss) included in earnings.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-04.13(h))

-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

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- Definition

The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of derivative instruments, including options, swaps, futures, and forward contracts, held at each balance sheet date, that was included in earnings for the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585

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