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	Mar. 31, 2014
Final Contractual Maturity [Abstract]
Due in 1 Year or Less	$ 3,399
Due in 1 to 5 Years	44,988
Due in 5 to 10 Years	115,034
Due after 10 Years	222,957
Total	386,378
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	28,822
Total	28,822
U.S. corporate bonds
Final Contractual Maturity [Abstract]
Due in 1 Year or Less	311
Due in 1 to 5 Years	15,816
Due in 5 to 10 Years	64,341
Due after 10 Years	1,359
Total	81,827
International corporate bonds
Final Contractual Maturity [Abstract]
Due in 1 Year or Less	0
Due in 1 to 5 Years	3,762
Due in 5 to 10 Years	11,923
Due after 10 Years	0
Total	15,685
Municipal bonds
Final Contractual Maturity [Abstract]
Due in 1 Year or Less	3,088
Due in 1 to 5 Years	25,410
Due in 5 to 10 Years	38,770
Due after 10 Years	192,776
Total	$ 260,044