Fair Value of Financial Assets and Liabilities, Final Contractual Maturity Dates of Debt Securities in Nuclear Decommissioning Fund (Details) $ in Thousands	Dec. 31, 2016 USD ($)
Final Contractual Maturity [Abstract]
Due in 1 Year or Less	$ 608
Due in 1 to 5 Years	44,215
Due in 5 to 10 Years	83,912
Due after 10 Years	47,276
Total	176,011
Government securities
Final Contractual Maturity [Abstract]
Due in 1 Year or Less	0
Due in 1 to 5 Years	9,158
Due in 5 to 10 Years	149
Due after 10 Years	22,658
Total	31,965
U.S. Corporate Bonds
Final Contractual Maturity [Abstract]
Due in 1 Year or Less	608
Due in 1 to 5 Years	28,375
Due in 5 to 10 Years	67,475
Due after 10 Years	9,314
Total	105,772
International Corporate Bonds
Final Contractual Maturity [Abstract]
Due in 1 Year or Less	0
Due in 1 to 5 Years	6,477
Due in 5 to 10 Years	10,525
Due after 10 Years	4,670
Total	21,672
Municipal Bonds
Final Contractual Maturity [Abstract]
Due in 1 Year or Less	0
Due in 1 to 5 Years	205
Due in 5 to 10 Years	5,763
Due after 10 Years	7,818
Total	13,786
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	2,816
Total	$ 2,816