Accounting for derivative instruments and hedging activities (Details 5) (Derivative instruments not designated as cash flow hedges, USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Unrealized gains and losses
Impact of derivative instruments on the consolidated statements of operations
Gains and losses for derivative instruments	$ 58,122	$ 4,239
Natural gas swaps | Realized (gains) and losses
Impact of derivative instruments on the consolidated statements of operations
Gains and losses for derivative instruments	4,815	2,476
Natural gas swaps | Unrealized gains and losses
Impact of derivative instruments on the consolidated statements of operations
Gains and losses for derivative instruments	1,795	2,883
Gas purchase agreements | Realized (gains) and losses
Impact of derivative instruments on the consolidated statements of operations
Gains and losses for derivative instruments	10,829
Gas purchase agreements | Unrealized gains and losses
Impact of derivative instruments on the consolidated statements of operations
Gains and losses for derivative instruments	57,877	678
Interest rate swaps | Realized (gains) and losses
Impact of derivative instruments on the consolidated statements of operations
Gains and losses for derivative instruments	(11,930)	(2,537)
Interest rate swaps | Unrealized gains and losses
Impact of derivative instruments on the consolidated statements of operations
Gains and losses for derivative instruments	(1,550)	678
Foreign currency forward contracts | Realized (gains) and losses
Impact of derivative instruments on the consolidated statements of operations
Gains and losses for derivative instruments	1,157	976
Foreign currency forward contracts | Unrealized gains and losses
Impact of derivative instruments on the consolidated statements of operations
Gains and losses for derivative instruments	$ 5,210	$ (3,436)