Selected Balance Sheet Data (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Accounts receivable, net [Abstract]
Accounts receivable	$ 65,605	$ 55,039	[1]
Less allowance for bad debts	(4,445)	(4,333)	[1]
Accounts receivable, net	61,160	50,706	[1]
Inventories [Abstract]
Inventories	15,052	19,685
Property, plant and equipment, net [Abstract]
Property, plant and equipment	2,226,209	2,193,812
Less accumulated amortization	(908,604)	(895,576)
Property, plant and equipment, net	1,317,605	1,298,236
Electric Plant [Member]
Property, plant and equipment, net [Abstract]
Property, plant and equipment	1,817,310	1,795,239
Natural Gas Plant [Member]
Property, plant and equipment, net [Abstract]
Property, plant and equipment	226,657	224,625
Common and Other Property [Member]
Property, plant and equipment, net [Abstract]
Property, plant and equipment	110,715	111,319
Construction Work in Progress [Member]
Property, plant and equipment, net [Abstract]
Property, plant and equipment	71,527	62,629
Materials and Supplies [Member]
Inventories [Abstract]
Inventories	6,191	6,172
Fuel [Member]
Inventories [Abstract]
Inventories	7,772	6,664
Natural Gas [Member]
Inventories [Abstract]
Inventories	$ 1,089	$ 6,849

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