Accounting for derivative instruments and hedging activities (Details 4) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Jun. 30, 2009
Changes in the accumulated other comprehensive income (loss)
Accumulated OCI balance at beginning of period	$ (1,383)	$ 255
Net unrealized losses on derivatives	245	272
Realized from OCI during the period	230	(449)
Accumulated OCI balance at end of period	(1,138)	527
Loss on discontinuation of swap contracts	(1,138)	527
Amortization of remaining loss on discontinuation of swap contracts	(230)	449
Interest rate swaps
Changes in the accumulated other comprehensive income (loss)
Accumulated OCI balance at beginning of period	(1,704)	(427)
Net unrealized losses on derivatives	15	721
Realized from OCI during the period	287	(360)
Accumulated OCI balance at end of period	(1,402)	(66)
Loss on discontinuation of swap contracts	(1,402)	(66)
Amortization of remaining loss on discontinuation of swap contracts	(287)	360
Natural gas swaps
Changes in the accumulated other comprehensive income (loss)
Accumulated OCI balance at beginning of period	321	682
Realized from OCI during the period	(57)	(89)
Accumulated OCI balance at end of period	264	593
Loss on discontinuation of swap contracts	264	593
Amortization of remaining loss on discontinuation of swap contracts	57	89
Discontinued cash flow hedge | Natural gas swaps
Changes in the accumulated other comprehensive income (loss)
Accumulated OCI balance at beginning of period			(5,100)
Realized from OCI during the period	(100)	(100)
Accumulated OCI balance at end of period			(5,100)
Loss on discontinuation of swap contracts			(5,100)
Amortization of remaining loss on discontinuation of swap contracts	$ 100	$ 100