Derivative instruments and hedging activities (Details 4) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Changes in the accumulated other comprehensive income (loss)
Accumulated OCI balance at beginning of period	$ (0.4)	$ (1.5)	$ (1.4)	$ (1.4)
Change in fair value of cash flow hedges	(0.1)	(0.3)	0.5	(0.8)
Realized from OCI during the period	0.2	0.1	0.6	0.5
Accumulated OCI balance at end of period	(0.3)	(1.7)	(0.3)	(1.7)
Interest Rate Swaps
Changes in the accumulated other comprehensive income (loss)
Accumulated OCI balance at beginning of period	(0.4)	(1.7)	(1.5)	(1.7)
Change in fair value of cash flow hedges	(0.1)	(0.3)	0.5	(0.8)
Realized from OCI during the period	0.2	0.2	0.7	0.7
Accumulated OCI balance at end of period	(0.3)	(1.8)	(0.3)	(1.8)
Natural Gas Swaps
Changes in the accumulated other comprehensive income (loss)
Accumulated OCI balance at beginning of period		0.2	0.1	0.3
Realized from OCI during the period		(0.1)	(0.1)	(0.2)
Accumulated OCI balance at end of period		$ 0.1		$ 0.1

Accumulated change, net of tax, in accumulated gains and losses from derivative instruments designated and qualifying as the effective portion of cash flow hedges. Includes an entity's share of an equity investee's Increase or Decrease in deferred hedging gains or losses.

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- Definition

Amount after tax of increase (decrease) in accumulated gain (loss) from derivative instruments designated and qualifying as the effective portion of cash flow hedges and an entity's share of an equity investee's increase (decrease) in deferred hedging gain (loss).

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