Selected Balance Sheet Data (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Accounts receivable, net [Abstract]
Accounts receivable	$ 49,827	$ 56,604
Less allowance for bad debts	(3,943)	(4,766)
Accounts receivable, net	45,884	51,838
Inventories [Abstract]
Inventories	18,007	25,703
Property, plant and equipment, net [Abstract]
Property, plant and equipment	2,115,487	2,066,426
Less accumulated depreciation	(872,049)	(858,728)
Property, plant and equipment, net	1,243,438	1,207,698
Electric Plant [Member]
Property, plant and equipment, net [Abstract]
Property, plant and equipment	1,725,905	1,684,537
Natural Gas Plant [Member]
Property, plant and equipment, net [Abstract]
Property, plant and equipment	217,159	213,665
Common and Other Property [Member]
Property, plant and equipment, net [Abstract]
Property, plant and equipment	109,588	113,597
Construction Work in Progress [Member]
Property, plant and equipment, net [Abstract]
Property, plant and equipment	62,835	54,627
Materials and Supplies [Member]
Inventories [Abstract]
Inventories	6,072	5,838
Fuel [Member]
Inventories [Abstract]
Inventories	8,934	9,335
Natural Gas [Member]
Inventories [Abstract]
Inventories	$ 3,001	$ 10,530