Derivative instruments and hedging activities (Details 5) (Derivative instruments not designated as cash flow hedges, USD $)
In Millions, unless otherwise specified

3 Months Ended

6 Months Ended

Jun. 30, 2013

Jun. 30, 2012

Jun. 30, 2013

Jun. 30, 2012

Unrealized gains and (losses)

Impact of derivative instruments on the consolidated statements of operations

Gains and losses for derivative instruments

$ (24.3)

$ 4.7

$ (36.9)

$ 62.0

Natural gas swaps | Unrealized gains and (losses)

Impact of derivative instruments on the consolidated statements of operations

Gains and losses for derivative instruments

1.1

0.5

0.7

1.4

Gas purchase agreements | Realized (gains) and losses

Impact of derivative instruments on the consolidated statements of operations

Gains and losses for derivative instruments

14.1

13.2

30.4

16.0

Gas purchase agreements | Unrealized gains and (losses)

Impact of derivative instruments on the consolidated statements of operations

Gains and losses for derivative instruments

(7.4)

1.2

(15.5)

59.1

Interest rate swaps | Realized (gains) and losses

Impact of derivative instruments on the consolidated statements of operations

Gains and losses for derivative instruments

4.0

1.1

6.6

2.2

Interest rate swaps | Unrealized gains and (losses)

Impact of derivative instruments on the consolidated statements of operations

Gains and losses for derivative instruments

(18.0)

3.0

(22.1)

1.5

Foreign currency forward contracts | Realized (gains) and losses

Impact of derivative instruments on the consolidated statements of operations

Gains and losses for derivative instruments

(10.8)

(3.1)

(13.3)

(15.0)

Foreign currency forward contracts | Unrealized gains and (losses)

Impact of derivative instruments on the consolidated statements of operations

Gains and losses for derivative instruments

$ 12.8

$ 7.7

$ 18.8

$ 12.9