Selected Balance Sheet Data (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012		Dec. 31, 2011
Accounts receivable, net [Abstract]
Accounts receivable	$ 53,444		$ 56,604
Less allowance for bad debts	(3,990)		(4,766)
Accounts receivable, net	49,454	[1]	51,838
Accounts receivable from affiliates	256
Inventories [Abstract]
Inventories	21,779		25,703
Property, plant and equipment, net [Abstract]
Property, plant and equipment	2,149,100		2,066,426
Less accumulated depreciation	(885,016)		(858,728)
Property, plant and equipment, net	1,264,084		1,207,698
Electric Plant [Member]
Property, plant and equipment, net [Abstract]
Property, plant and equipment	1,744,066		1,684,537
Natural Gas Plant [Member]
Property, plant and equipment, net [Abstract]
Property, plant and equipment	219,630		213,665
Common and Other Property [Member]
Property, plant and equipment, net [Abstract]
Property, plant and equipment	110,494		113,597
Construction Work in Progress [Member]
Property, plant and equipment, net [Abstract]
Property, plant and equipment	74,910		54,627
Materials and Supplies [Member]
Inventories [Abstract]
Inventories	6,369		5,838
Fuel [Member]
Inventories [Abstract]
Inventories	6,960		9,335
Natural Gas [Member]
Inventories [Abstract]
Inventories	$ 8,450		$ 10,530

[1] Accounts receivable, net includes $256 due from affiliates as of Sept. 30, 2012.