Fair Value of Financial Assets and Liabilities, Final Contractual Maturity Dates of Debt Securities in Nuclear Decommissioning Fund (Details) $ in Thousands	Dec. 31, 2015 USD ($)
Final Contractual Maturity [Abstract]
Due in 1 Year or Less	$ 153
Due in 1 to 5 Years	19,056
Due in 5 to 10 Years	81,410
Due after 10 Years	64,854
Total	165,473
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	21,356
Total	21,356
U.S. Corporate Bonds
Final Contractual Maturity [Abstract]
Due in 1 Year or Less	0
Due in 1 to 5 Years	16,005
Due in 5 to 10 Years	51,384
Due after 10 Years	(2,113)
Total	65,276
International Corporate Bonds
Final Contractual Maturity [Abstract]
Due in 1 Year or Less	0
Due in 1 to 5 Years	2,787
Due in 5 to 10 Years	9,382
Due after 10 Years	632
Total	12,801
Municipal Bonds
Final Contractual Maturity [Abstract]
Due in 1 Year or Less	153
Due in 1 to 5 Years	264
Due in 5 to 10 Years	17,814
Due after 10 Years	33,358
Total	51,589
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	2,830
Due after 10 Years	0
Total	2,830
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	11,621
Total	$ 11,621