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	Jun. 30, 2014
Final Contractual Maturity [Abstract]
Due in 1 Year or Less	$ 2,915
Due in 1 to 5 Years	46,114
Due in 5 to 10 Years	118,444
Due after 10 Years	225,824
Total	393,297
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	30,545
Total	30,545
U.S. corporate bonds
Final Contractual Maturity [Abstract]
Due in 1 Year or Less	307
Due in 1 to 5 Years	15,358
Due in 5 to 10 Years	66,131
Due after 10 Years	2,434
Total	84,230
International corporate bonds
Final Contractual Maturity [Abstract]
Due in 1 Year or Less	0
Due in 1 to 5 Years	3,854
Due in 5 to 10 Years	12,578
Due after 10 Years	0
Total	16,432
Municipal bonds
Final Contractual Maturity [Abstract]
Due in 1 Year or Less	2,608
Due in 1 to 5 Years	26,902
Due in 5 to 10 Years	36,195
Due after 10 Years	162,801
Total	228,506
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	3,540
Due after 10 Years	5,794
Total	9,334
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	24,250
Total	$ 24,250