Fair Value of Financial Assets and Liabilities, Final Contractual Maturity Dates of Debt Securities in Nuclear Decommissioning Fund (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013
Final Contractual Maturity [Abstract]
Due in 1 Year or Less	$ 5,524
Due in 1 to 5 Years	70,219
Due in 5 to 10 Years	133,785
Due after 10 Years	175,673
Total	385,201
Government Securities
Final Contractual Maturity [Abstract]
Due in 1 Year or Less	0
Due in 1 to 5 Years	2,793
Due in 5 to 10 Years	11,211
Due after 10 Years	35,698
Total	49,702
U.S. Corporate Bonds
Final Contractual Maturity [Abstract]
Due in 1 Year or Less	1,734
Due in 1 to 5 Years	39,998
Due in 5 to 10 Years	81,716
Due after 10 Years	11,123
Total	134,571
International Corporate Bonds
Final Contractual Maturity [Abstract]
Due in 1 Year or Less	0
Due in 1 to 5 Years	3,115
Due in 5 to 10 Years	14,588
Due after 10 Years	1,000
Total	18,703
Municipal Bonds
Final Contractual Maturity [Abstract]
Due in 1 Year or Less	3,790
Due in 1 to 5 Years	24,313
Due in 5 to 10 Years	26,270
Due after 10 Years	127,852
Total	$ 182,225