Fair Value of Financial Assets and Liabilities, Final Contractual Maturity Dates of Debt Securities in Nuclear Decommissioning Fund (Details) $ in Thousands	Mar. 31, 2016 USD ($)
Final Contractual Maturity [Abstract]
Due in 1 Year or Less	$ 151
Due in 1 to 5 Years	21,970
Due in 5 to 10 Years	89,910
Due after 10 Years	63,393
Total	175,424
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	3,144
Due after 10 Years	20,069
Total	23,213
U.S. corporate bonds
Final Contractual Maturity [Abstract]
Due in 1 Year or Less	0
Due in 1 to 5 Years	18,909
Due in 5 to 10 Years	56,102
Due after 10 Years	(4,288)
Total	70,723
International corporate bonds
Final Contractual Maturity [Abstract]
Due in 1 Year or Less	0
Due in 1 to 5 Years	2,795
Due in 5 to 10 Years	11,505
Due after 10 Years	3,043
Total	17,343
Municipal bonds
Final Contractual Maturity [Abstract]
Due in 1 Year or Less	151
Due in 1 to 5 Years	266
Due in 5 to 10 Years	16,323
Due after 10 Years	33,162
Total	49,902
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	2,836
Due after 10 Years	0
Total	2,836
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	11,407
Total	$ 11,407