Financial Instruments And Investments (Company's Decommissioning Trust Funds And Investments In Debt Securities) (Details) - USD ($) $ in Thousands		Mar. 31, 2020	Dec. 31, 2019
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value		$ 294,501	$ 325,998
Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value		294,501	325,998
Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value		0	0
Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value		0	0
Cash
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value		2,041	1,731
Cash | Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value		2,041	1,731
Cash | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value		0	0
Cash | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value		0	0
Debt Securities
Financial Instruments and Investments [Line Items]
Trading Securities		1,565	1,587
Debt Securities | Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Trading Securities		0	0
Debt Securities | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Trading Securities		0	0
Debt Securities | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Trading Securities		1,565	1,587
Equity Securities
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value		292,460	324,267
Equity Securities | Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value		292,460	324,267
Equity Securities | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value		0	0
Equity Securities | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value		0	0
Equity Securities | Domestic Equity Securities
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	[1]	270,522	295,065
Equity Securities | Domestic Equity Securities | Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	[1]	270,522	295,065
Equity Securities | Domestic Equity Securities | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	[1]	0	0
Equity Securities | Domestic Equity Securities | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	[1]	0	0
Equity Securities | International Equity Securities
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value		21,938	29,202
Equity Securities | International Equity Securities | Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value		21,938	29,202
Equity Securities | International Equity Securities | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value		0	0
Equity Securities | International Equity Securities | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value		0	0
Exchange Traded Funds | Domestic Equity Securities
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value		$ 151,900	$ 148,100

[1] (a) Includes $151.9 million and $148.1 million, as of March 31, 2020 and December 31, 2019, respectively, held in exchange traded funds with underlying investments in debt securities that meet the definition of equity securities with readily determinable fair values.