Summary of Significant Accounting Policies (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Balance Sheets
Long-term debt less current maturities	$ 2,796,465	$ 3,199,088
Cash Flows from Operating Activities
Deferred income taxes	249,296	187,023	117,952
Deferred investment tax credit	52,542	41,579	58,240
Income tax receivable	(133,094)	(4,043)	3,983
Accrued taxes	6,059	12,736	8,085
As previously reported
Balance Sheets
Long-term debt less current maturities		3,160,219
Cash Flows from Operating Activities
Deferred income taxes		228,602	176,192
Accrued taxes and income tax receivable		8,693	12,068
As previously reported | Palo Verde Sale Leaseback
Balance Sheets
Long-term debt less current maturities		38,869
Reclassifications to conform to current year presentation
Balance Sheets
Long-term debt less current maturities		38,869
Cash Flows from Operating Activities
Deferred income taxes		(41,579)	(58,240)
Deferred investment tax credit		41,579	58,240
Accrued taxes and income tax receivable		(8,693)	(12,068)
Income tax receivable		(4,043)	3,983
Accrued taxes		12,736	8,085
Reclassifications to conform to current year presentation | Palo Verde Sale Leaseback
Balance Sheets
Long-term debt less current maturities		$ (38,869)