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	Sep. 30, 2014
Final Contractual Maturity [Abstract]
Due in 1 Year or Less	$ 1,110
Due in 1 to 5 Years	54,332
Due in 5 to 10 Years	121,622
Due after 10 Years	223,473
Total	400,537
Government securities
Final Contractual Maturity [Abstract]
Due in 1 Year or Less	0
Due in 1 to 5 Years	0
Due in 5 to 10 Years	0
Due after 10 Years	29,726
Total	29,726
U.S. corporate bonds
Final Contractual Maturity [Abstract]
Due in 1 Year or Less	303
Due in 1 to 5 Years	15,878
Due in 5 to 10 Years	62,985
Due after 10 Years	82
Total	79,248
International corporate bonds
Final Contractual Maturity [Abstract]
Due in 1 Year or Less	0
Due in 1 to 5 Years	4,266
Due in 5 to 10 Years	13,347
Due after 10 Years	0
Total	17,613
Municipal bonds
Final Contractual Maturity [Abstract]
Due in 1 Year or Less	807
Due in 1 to 5 Years	34,188
Due in 5 to 10 Years	41,744
Due after 10 Years	164,168
Total	240,907
Asset-backed securities
Final Contractual Maturity [Abstract]
Due in 1 Year or Less	0
Due in 1 to 5 Years	0
Due in 5 to 10 Years	3,546
Due after 10 Years	5,801
Total	9,347
Mortgage-backed securities
Final Contractual Maturity [Abstract]
Due in 1 Year or Less	0
Due in 1 to 5 Years	0
Due in 5 to 10 Years	0
Due after 10 Years	23,696
Total	$ 23,696