Selected Balance Sheet Data (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011
Accounts receivable, net [Abstract]
Accounts receivable	$ 55,039		$ 56,604
Less allowance for bad debts	(4,333)		(4,766)
Accounts receivable, net	50,706	[1]	51,838
Accounts receivable from affiliates	586
Inventories [Abstract]
Inventories	19,685		25,703
Property, plant and equipment, net [Abstract]
Property, plant and equipment	2,193,812		2,066,426
Less accumulated depreciation	(895,576)		(858,728)
Property, plant and equipment, net	1,298,236		1,207,698
Electric Plant [Member]
Property, plant and equipment, net [Abstract]
Property, plant and equipment	1,795,239		1,684,537
Natural Gas Plant [Member]
Property, plant and equipment, net [Abstract]
Property, plant and equipment	224,625		213,665
Common and Other Property [Member]
Property, plant and equipment, net [Abstract]
Property, plant and equipment	111,319		113,597
Construction Work in Progress [Member]
Property, plant and equipment, net [Abstract]
Property, plant and equipment	62,629		54,627
Materials and Supplies [Member]
Inventories [Abstract]
Inventories	6,172		5,838
Fuel [Member]
Inventories [Abstract]
Inventories	6,664		9,335
Natural Gas [Member]
Inventories [Abstract]
Inventories	$ 6,849		$ 10,530

[1] Accounts receivable, net includes $586 due from affiliates as of Dec. 31, 2012.