Selected Balance Sheet Data (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Accounts receivable, net [Abstract]
Accounts receivable	$ 63,520	$ 66,789
Less allowance for bad debts	(4,570)	(4,722)
Accounts receivable, net	58,950	62,067
Inventories [Abstract]
Inventories	24,481	31,337
Property, plant and equipment, net [Abstract]
Property, plant and equipment	4,704,978	4,616,344
Less accumulated depreciation	(1,773,702)	(1,754,588)
Property, plant and equipment, net	2,931,276	2,861,756
Electric Plant [Member]
Property, plant and equipment, net [Abstract]
Property, plant and equipment	4,428,802	4,379,208
Construction Work in Progress [Member]
Property, plant and equipment, net [Abstract]
Property, plant and equipment	276,176	237,136
Materials and Supplies [Member]
Inventories [Abstract]
Inventories	18,932	18,129
Fuel [Member]
Inventories [Abstract]
Inventories	$ 5,549	$ 13,208