Fair Value of Financial Assets and Liabilities, Final Contractual Maturity Dates of Debt Securities in Nuclear Decommissioning Fund (Details) $ in Thousands	Mar. 31, 2017 USD ($)
Final Contractual Maturity [Abstract]
Due in 1 Year or Less	$ 354
Due in 1 to 5 Years	47,926
Due in 5 to 10 Years	91,082
Due after 10 Years	42,197
Total	181,559
Government securities
Final Contractual Maturity [Abstract]
Due in 1 Year or Less	0
Due in 1 to 5 Years	1,100
Due in 5 to 10 Years	3,017
Due after 10 Years	23,082
Total	27,199
U.S. corporate bonds
Final Contractual Maturity [Abstract]
Due in 1 Year or Less	354
Due in 1 to 5 Years	38,741
Due in 5 to 10 Years	74,617
Due after 10 Years	15,087
Total	128,799
Non U.S. corporate bonds
Final Contractual Maturity [Abstract]
Due in 1 Year or Less	0
Due in 1 to 5 Years	8,085
Due in 5 to 10 Years	13,443
Due after 10 Years	4,028
Total	25,556
Municipal bonds
Final Contractual Maturity [Abstract]
Due in 1 Year or Less	0
Due in 1 to 5 Years	0
Due in 5 to 10 Years	5
Due after 10 Years	0
Total	$ 5